Developing Children's Book Characters
Jack Heffron in The Writer's Ideas Book says:
There's an old rule about writing that says readers won't care what happens in a story if they don't care WHO it happens to.
So, once you have the glimmering of an idea about the story you intend to write, you need to decide on your cast of characters. Plot and character should go hand in hand.
Choosing your Characters
As you are a children's writer, you will almost certainly have several children in your cast. Have you noticed that children are always in a hurry to grow up? This may be why they like to read about children a little older than themselves. For your main character, choose someone at the top of your target age range, so if you are aiming your story at 7-11 year olds, an 11- or 12-year-old is an ideal hero, perhaps with a sibling of 7 or 8 to keep the lower end of the range interested.
A mix of boys and girls should be used in stories, and if it is a contemporary story a multi-cultural group of characters to reflect the reality of today's world.
You will probably also need some adults. Will your main character be living at home with his family? If so you need to think about his parents and other relatives who play a part in his life. You need to decide on their background. Are they well-off? Poor? What does the father do for a living? Does the mother work? How does your character get on with his parents? Do they live in a town or in the country? If you are writing a fairy tale, are your characters princes and princesses or are they humble folk? If you are writing a fantasy decide whether your character can do magic or not. If it is science fiction, you will have to create an environment for them. All these factors shape your main character's background.
You need an antagonist, someone who can frustrate the hero's aim, who can stand in his way and who will push against him, giving the story conflict and tension. It can be another child or it can be an adult. It need not be an out and out villain - but it can be; the choice is yours. If you are writing a fantasy you have plenty of opponents to choose from - witches, wizards, dragons and any of the fairy folk such as elves, goblins, trolls, or fairies. A good use of your antagonist is to let him discover the hero's weak point and use it against him in any way he can, as often as he can.