The most important part of the story are the characters. Without characters for the reader to root for and live vicariously through, there is no point to the story. So crafting compelling, dynamic, interesting characters should always be in the back of your mind. For a kids story the main characters need to be human (in the flawed not anatomical sense) and they need to have the sensibilities of a child. When you are picking the sex for the character there are two things to consider. Boys will not read girls books. They define girls book as books with female protagonists. On the other hand, teachers and librarians are always looking for more books with strong female characters. Also, a majority of children author, editors, and publishers (or imprint heads) are women. This tends to skew books towards female characters as well. But ultimately you have to write the character that calls to you. Normally, it tells you if it’s male or female. Your primary characters are the ones that must show growth and development during the story arc.
Now that you have your primary characters or characters, you have to start building the people in your protagonist’s world. These can be adults as well as other kids. Sometimes it includes pets. These characters will not be as fully developed as your primary character, but they still might show some growth. You might enjoy making up complex back stories for these characters, but you almost certainly will not work it into your book. Most likely it would be distracting and irrelevant to the story arcs.
And finally after you’re done building all these characters and working them into your story, you have to figure out which ones are irrelevant. You may find that two of your characters are similar and do the same things. Feel free to combine them into one. Others have little or no point at all. Save them for future books.
This is a very cursory discussion on character. Lots of books give more detailed explanations. If you have any questions, ask me. I’ll be happy to go into anything in greater detail.
Now I’m off to work at my other job where I have no internet connection but lots and lots of slush.