Create a short outline for your book, and keep it age appropriate. Books written for children 5 and under should have plenty of room for an artist to interpret the material and illustrate it. Children older than 5 years old are more capable of understanding a story with little or no illustrative assistance.
Create stories and characters that children can relate to. Try to stick with universal issues. Use your stories as a means of teaching without preaching. Be aware of how you write. You don't want to talk down to your readers because they're young. In fact, keep the writing level slightly above the reading level to challenge children to expand their reading and comprehension abilities.
Illustrate your books if you have the ability to do it. If not, children's book publishers have artists on staff that can illustrate for you. Feel free to suggest ideas you may have for illustrations.
Study the markets before you pitch your children's book. The Writer's Digest Children's Book Market is a good place to start (see Resources below). You can also consult an online market database for children's books (see Resources below). Target the markets that accept the type of material you've written, and follow their submission guidelines when sending your manuscript for review.
Don't wait for a response from your first submission before you begin another project. Start writing your next book while you wait. If you feel the first book deserves a sequel, this may be a good time to start it. Series books are popular in children's fiction, and a solid central theme and likable characters could very well lead to a contract to do several children's books with the same publisher.