Symbolism, Imagery, Allegory
On one level, The Boxcar Children is about the fantasy of running away, which is something children are never supposed to do—but in every other way, the book is about what children are supposed to do. In that sense, the novel is an allegory, i.e., a story that has a moral message under its surface.
With The Boxcar Children, the message is about how to lead a good life. Think of that message as medicine, except instead of being gussied up with sugar and food coloring to make it go down more easily, it's dressed up like a fun little adventure story. The fantasy of running away makes the message fun to read—if the author had just made a list of rules about how to live, that wouldn't have been nearly as entertaining. We're pretty sure kids wouldn't still be reading that list all these years later, though The Boxcar Children remains a classic.
One thing you may have noticed about Henry, Jessie, Violet, and Benny is that they don't have a ton of personality. (There's more on this over in the "Characters" section, so check it out.) They have differences, sure—Jessie is the caring one, and Benny is always hungry—but as individuals, we don't really know what makes each person tick. That's because the Alden children are designed to be role models more than they're meant to be nuanced characters. They embody important values like self-reliance, being hard-working, and living a moral life, while their actions are meant to serve as little lessons to the reader about things like hygiene and generosity.
Minus the running-away part, the Boxcar Children are pretty perfect. Heck, they only run away because they're orphaned and don't want to live with their "nasty" grandpa—so it's not like they even run off for particularly immoral reasons. Through and through, these kids are good eggs … just like Warner wants her readers to be.