The best way to get started with this prompt is to identify the theme you want to explore and then the specific three characters and/or character relationships you believe support your interpretation of the theme.

From there, the simplest organization of your essay is fairly straight-forward:

Paragraph 1 -- Introduction

Paragraph 2 -- Character (or character relationship) A

Paragraph 3 -- Character (or character relationship) B

Paragraph 4 -- Character (or character relationship) C

Paragraph 5 -- Conclusion

In paragraphs two through four, you will describe how this theme is developed because of this character (or relationship) using specific examples (and I assume quotations) from the book.

My advice would be to begin writing these paragraphs first, and once they are drafted, come back to draft your introduction and conclusion.

You have some great information brainstormed.  The key point you are missing is a theme statement.  Once you get this down it can serve as the thesis for the whole paper.  Then, the examples you use to prove it will come from character and character relationships.

Follow the link below to a thread on how to write a theme statement.

Skip steps 1&2, it sounds like you are working from the subject: "children's attitudes about growing up" - so when you get to #3, ask, "What is the author trying to say about children's attitudes associated with growing up?  The "cause-effect machine" should work really well with this, and your examples are naturally going to come from the characters and their relationships with one another.  Perhaps each body paragraph could support your theme statement from a different character's point of view.  Just be sure to have a theme statement that you can prove with all three characters.  *If you are still struggling - post your theme statement (or some attempts) and get feedback there first.

Good luck.