Think About Your Readers
When you're writing for children, the age group you're targeting will be determined by the age of your main character. Generally, children want their heroes and heroines to be a year or two older than they are, never younger.
Unlike adults, who often yearn to be children again, kids are eager to grow up and carry no nostalgia for the phases they've already lived through. A seven-year-old isn't interested in a five-year-old main character, and a ten-year-old isn't interested in a seven-year-old main character. If the hero of your story is seven, you're writing for the five- to six-year-old reader.
Although they like to read about children their age or a little older, most kids aren't terribly interested in reading about adults, especially in fiction. The best children's fiction keeps adult characters to a minimum. The focus is always on the child in the story and how he copes with the problems he encounters.
Diminutive terms such as “Little Sally” or “Little Jake” can turn children off your book. Kids look forward to being big enough to do the things they read about; they don't want their main characters to be “little.”
Older children might part ways according to gender when it comes to interest levels in various subjects; boys may be more interested in sports stories, and girls may be more interested in romances, for instance. However, the trend in children's books, especially for younger readers, is to try to appeal to both boys and girls. Many of today's children's books feature both boy and girl lead characters, and many of them weave diverse, multicultural traits into their plots.
Many beginning writers make two common errors in their children's stories. Either they write from an adult's point of view, or they talk down to their readers. Children are pretty astute when it comes to judging tone, even in the written word, and they'll cast aside any book at the first hint of preachiness or condescension.
Once you've determined the age group your story is aimed at, spend some time with children that age to get a feel for what they know, what they think about, and what they like, and keep those characteristics in mind when you're writing.