Keep it simple. Start with a conflict that kids understand. Something as complicated as "The Da Vinci Code" won't work for young children. Kid's fiction is usually divided into three groups: elementary, middle school and young adult. Lemony Snicket books and Captain Underpants are great for elementary level. Middle school examples would include Madeline Engel or Harry Potter. Young adults tend to enjoy darker adventures such as the Twilight series.
Understand your target age group. Make sure your material is age-appropriate, avoiding love scenes, expletives and gore in favor of action and suspense suitable for younger children. No matter what age group you're writing for, avoid patronizing language. Kids are very savvy and will be turned off if they sense you're talking down to them.
Keep it moving. The pace should be quick from one scene to the next. Cliffhangers at the ends of chapters keep kids interested and make the book difficult to put down.
Find a critique partner or group. Whether online or in person, other readers give you valuable perspective, and seasoned writers can mentor you.
Build three-dimensional characters. The kids in your story should be easy for children to relate to. Give them hopes, fears and flaws that other kids understand. Children will not likely fall for a perfect character or a hopelessly flawed one.
Respect your audience. Never make them think you are an adult who is writing "just a kid's story." Believe in your characters and their situations, and your audience will, too.