I guess my first question is, who is the target audience? Children's chapter books are usually shorter than adult, so introducing during the first chapter should be plenty. As for the hero method, it may still apply, except you need to think in terms of the environment. For example, in a classroom, the journey experience might be coming up with a science project for a fair. The antagonist may be a bully who ruins his project, etc. If this targetted for adults, the circumstances may be more harsh, but they should still have a basic progression of the main character (hero) growing through a series of events. If not the character growing, maybe it is we who grow by understanding. But there should be a step by-step-process none-the-less that would fall into a similar pattern.
As for introducing many different characters, it depends on why you feel the need to have so many. If you are wanting to intertwine the tales, it might be a good idea to write through one character at a time and then see how you can connect them. It is always easiest to focus on one point of view, so if you have a major need to change pov, you might want to make each child's tale a different short story, and then perhaps have one story at the end converging them all, if you still need them together, from the pov of one who would make the most impact (grow the most) from being the main focus/perspective. Otherwise, one character is the least chaotic to follow, and will flow the smoothest. He can bump into other characters,etc, and share his thoughts about them, but we really don't need alot of background info on them all.
Hope this helps
