I would like to offer something different from what missy575 has posted. In addition to being a tomboy, Scout is (paradoxically) a daddy's girl. One of the perfect moments for her, a moment represented in several places in the novel, involves sitting on her father's lap while he reads to her. This perfect moment seems to me to fit very well with the idea of a children's book.
I'm not completely convinced that Harper Lee's novel To Kill a Mockingbird is a story about loss of innocence, but you certainly can read it that way. You may be interested in tracking, for example, how Scout matures in her understanding of stories. At first she takes the stories about Boo Radley literally. At the end of the novel, she's again sitting in her father's lap while he reads to her, but I get the sense that she's grown up at least a little. Now, perhaps, she enjoys stories as stories. She has developed in her ability to distinguish between fantasy and reality while keeping the ability to enjoy fantasy (or stories).
Scout is a true tom-boy. Most girls who are experience this are imaginative and tough. If you take a look the first experiences she has in the book, she is acting out plays or books, climbing into a treehouse, meeting a new kid, playing with boys and being able to read at her pleasure.
The first thing I would write about her perfect world would be a word picture about what it's like to be a tom-boy, and specifically for children, the tree-climbing, and the scary story about the man down the street would be fun to see the innocence in her. If your project has to do with the loss of innocence, I imagine you want to show she has innocence in the beginning and looses it along the way.