I recently visited my parents, and while there, I spent some time sorting through the shelves full of my old books that I'd left there when I moved out. I sadly had to let a lot of them go to the local thrift shop (it's much too far for me to drive a car-full of books over to my own place), but there were a lot of books that still looked too good for me to give away just yet. Some of them were history or philosophy books I'd gotten into as a teenager, but some of them were kids' books from my childhood. And some of those were interesting enough that I reread them right then and there. I ended up rereading several children's books that I hadn't read since I was a kid myself, or at least a teenager. I enjoyed them as a kid (I remembered them very fondly), and I enjoyed them when rereading them now.
A lot of them were decades older than me - some from my parents' own childhood, and some from even before their time. They were very different from the more adult- or at least young-adult-targeted books I'd read more recently, and that made me think more about what made them so different and how they impacted me in my childhood.
Beyond just the books I reread, this brought back so many memories of reading as a kid. We didn't have a TV, and my parents weren't early adopters of the Internet, so I spent so many happy hours curled up on the couch or sprawled out by the sliding door window reading a good book. We had two whole walls covered with bookshelves, and I read... okay, not all of them (my parents had large collections of their own that didn't all interest me, and I didn't keep up with my sister's interest in animal-focused books) but most of them.
But the particular books that I reread - and so, the ones I can dig into detail on beyond my childhood memories - are, with the year of publication:
First, there were four books set more or less in the contemporary world:
The Boxcar Children, by Gertrude Chandler Warner (1924)
Homer Price, by Robert McCloskey (1943)
The Mystery of the Empty House (aka Secret of the Old Post Box), by Dorothy Sterling (1960)
The Marvelous Inventions of Alvin Fernald, by Clifford B. Hicks (1962)
Second, there were three books set in the midst of specific historical events:
Snow Treasure, by Marie McSwigan (1942), set in 1940 during the Nazi invasion of Norway.
The Little Riders, by Margaretha Shemin (1963), set in 1944 during the Nazi occupation of Holland.
When The Dikes Broke, by Alta Halverson Seymour (1958), set in 1953 during the Great Flood of Holland.
One of the first things I noticed about these books - even before I opened the front cover - was how short they were. All these seven were between a hundred and two hundred pages, and usually in largish print. I haven't tried to find their word count, but I'd be surprised if any of them were more than thirty thousand words. But, I was surprised to notice this because they seemed so much larger and longer in my childhood memories. A lot happens in them. The authors here move from scene to scene quickly; a whole abortive baseball game takes up maybe two short pages even though it feels fully described not summarized. We don't get any soliloquies, and few long speeches; characters' inner thoughts don't get much room to play. For that matter, most supporting characters aren't more than one- or two-dimensional. But I didn't notice those lacks as a kid. When I was rereading the books now, I didn't really think they were flaws. The author packed into a short book what would've taken maybe three times as long in any other style. Even a slow young reader can finish one of these in a day. I read quickly as a kid, so I could finish one of these books in part of an afternoon, and even I liked that.
What's more, it feels like even more happens in the books than actually does happen "onstage." The authors freely state outright what a character normally does, and allude to past adventures or otherwise offstage adventures. Traditionally, people advise authors to show not tell. These statements about backstory or offstage stories at least bend that a lot - but it works for kids. As a kid, I think I happily filled in everything that was missing without any problem - in fact, I don't remember making any conscious effort to do so. I'm reminded of when someone said online (I forget where) that, when rereading The Lion, The Witch, and the Wardrobe, they were surprised to realize that Edmund doesn't break the White Witch's wand onstage. Peter recounts it in retrospect. But, this person said, they remember picturing it so vividly as a kid! I felt the same way about so many things mentioned about so many characters. My imagination must've been more active with smaller prompts.
In fact, I think clearly-stated backstory can help kids find their bearings in an imaginary world. I remember so many wild headcanons I created as a kid because I misunderstood some detail of the world. My parents once mentioned potentially "calling 911 to" somewhere. Of course, they meant getting the police there - but, being about six at the time, I took it to mean there was some way to dial "911" on the phone and get an emergency-level phone connection to a particular place. I don't remember specifically misunderstanding details from a book to this degree, but I'm sure it happened. So, when your readers might misunderstand things to this degree, it can help to specifically spell out that (say) Shoeie and Alvin call each other "Great Athlete" and "Old Man" because it's their longtime exaggerated nicknames for each other.
Length does matter to some extent. Take Beverly Cleary, for example - I didn't reread any of her books this year, but I've got very fond memories of reading them as a kid. She also wrote kid-focused slice-of-life books around 200 pages in length, and a lot of them - the Klickitat Street series about Ramona and Beezus and Henry Huggins - were set in the same world with the same kids. (Her books were more clearly slice-of-life, actually, event following event, without action rising to a climax. Or, at least, it was only an emotional climax.) As a kid, I think I enjoyed reading more about the same characters. I think I did have a clearer image of Cleary's Beezus or Ramona or Henry Huggins in my mind than of Homer Price or Alvin Fernald who appeared in fewer books, but I don't think I consciously appreciated that, and I know it didn't hold me back from enjoying short books.
These books might have been written as realistic, but they were a different world to me as a kid.
All the kids in all these books had freedom - not just the freedom to solve whatever difficulties were in the plot on their own, but the freedom to walk or bike around town and largely arrange their lives on their own. I'm not saying they had that freedom in every instance, but they usually did to a large extent. I don't think I made this connection as a kid, but thinking about it now, that freedom is essentially what lets these stories exist.
The extreme of this freedom is The Boxcar Children, where the children are orphaned and literally living by themselves in the forest for most of the book. But even when a story involves kids who are living in town with parents - even when the parents are involved with the kids' lives - the kids have the freedom to manage the story on their own. Pat (in Mystery of the Old House) can get invited to a pickup baseball game a day after her family moved to town, and spend half the summer hanging around the neighbors' historic house, without her parents being surprised. Homer Price can even be left to manage his uncle's restaurant without anyone thinking it remarkable.
In my childhood, I think I just accepted this level of autonomy as the rules of another world. Just like kids in a fantasy book might have magic, kids in these books had autonomy. Looking back on this from the modern world, or even from my own childhood, this level of kid autonomy does seem almost as fantastic as the magic of a fantasy novel. But as far as I can tell now, when these books were written between 1924 and 1963, it was actually realistic. The other world I was reading about in my childhood was actually real.
As a kid, I think I did want this on some level, but on the same level that I wanted to visit Mars or Narnia. I was happy with the wall-to-wall packed bookcases, with riding my bike much closer to home than anyone from the books, and with the friends I did see when their parents or mine gave us a ride. But just the same - looking back - I think this vision of another world did widen my horizons and prepare me for when I did grow up more and gain more autonomy myself.
Yes, the kids' autonomy was coupled with their parents and the whole adult world still existing around them in the background. But, it was in the background. They'd step around it, and sometimes it'd intrude on events to (say) ground our young protagonist, but it was not what was focused on. Just like Sanderson's Laws of Magic remind fantasy writers that they can't use unexplained magic to solve their protagonist's problems, in the same way, the unexplained adult world isn't allowed to solve our young protagonist's problems. Sanderson is totally fine with unexplained magic causing problems, and similarly, the unexplained adult world is totally free to impede the kids. But it can only help the kids at the denouement, after they've solved the plot by their own spunk and skill.
And these problems the kids solve are real problems. Even apart from the three books set during WWII and the Dutch flood, Homer catches robbers and Alvin finds a vanished neighbor and Pat finds valuable historical documents and saves her neighbors' house. These are adult-scale problems. Reading them as an adult, I can detect the careful setup that makes them possible for our young protagonists to solve - but it's still a very real achievement for our young protagonists.
The historically-set books generally follow this same pattern, too. For one reason or another, there's a problem the adults aren't in the right place to solve - whether because they'd be more suspicious to the Nazis or because they're busy themselves - and our kid protagonists need to step in. I remember other books focusing on the kids just observing a major historical event, but even then, there were smaller problems spun off for them to surmount.
I have kept reading children's books to some extent as I've grown up, but I wouldn't say I've really followed modern kids' books. The "middle-grade" books I've read are longer, and usually fantasy, like the (excellent) Down the Mysterly River or Splendors and Glooms. It feels like they're meant for older kids... though my own childhood reading was so mixed-up that I can't really feel what ages they're intended for. So, it's hard to compare them directly to the books I read in my own childhood.
I seem to have the impression (for what it's worth) that, as childhood has changed, more modern children's books had to accept a smaller horizon when set in the real world. Perhaps I'm wrong; I usually didn't notice the publication dates on books when I was growing up. If I had time to study it, it would be interesting to see how these changes in childhood affected children's authors who were writing both before and after these changes. The two long series I remember from my childhood that were clearly written and set in the 1990's - Magic Tree House and Animorphs - both used magic or sci-fi technology to keep their young protagonists from getting in trouble with their parents. I don't know whether this was a genre-wide trend, but it would be interesting if it was.
It'd be even more interesting to see how they affected continuing series that started out with kids in roughly contemporary times, like Beverly Cleary's Klickitat Street series written between 1950-1999, or Donald Sobol's Encyclopedia Brown series between 1963-2012. Perhaps this's one reason why Cleary, at least, was focusing on younger kids in her later books? (Or at least it seems that way from the titles; I didn't read them as a kid.) Ten-year-old Ramona can have more parental limits than older kids did in previous books, without clearly either breaking continuity or breaking from the real world.
Maybe the genre I've been talking about here has disappeared. I'm sure it's changed a lot. I'm sure it'd changed a lot even before I was reading these books; they're far older than me, and most of my childhood friends were watching television at least as much as reading. But I hope something has kept the essence of what I've been talking about here, because I liked it a lot, and I think it gave me something important in my childhood.
Fun, I'm 64 and yeah those books about kids doing stuff, was what I did. It was all free range kids back then. Which was great. I don't think I know any of those books, except maybe the boxcar kids, and certainly Encyclopedia Brown. I read all of those in the local library and then went right to Sherlock Holmes. I soon found the YA sci-fi section and gobbled up all of that. Heinlein became my total fav.