Spend time among children and immerse yourself in children's literature, both the classics and modern books. Talk to parents and teachers, if possible, as they often have an insight into what children like to read. Cast your mind back to your own childhood as well, and remember the books you liked to read. Though fashions in children's literature do change, some factors stay the same--for instance children usually like to identify with the main character in the story.
Look in the book stores and libraries, studying particularly the books aimed at the age group you want to write for. Be clear in your mind about the age group as this is crucial. Check the average length of the books aimed at your chosen age group. Remember that if you are writing for children under 5 years old, you will need illustrations. Do not attempt to do this yourself, unless you are an expert.
Write an outline of the book--this is essential groundwork. Establish the main character, including the gender, the time-line and the plot. Begin writing your story, launching as quickly into the action and the plot as possible. Write fairly short sentences and paragraphs. Children will not usually stay with the book if they are not compelled to turn the pages.
Study the publishing world, particularly "The Writer's Market" and writing magazines. Approach publishing houses that handle children's fiction. Write a cover letter, giving basic information about yourself. Include a synopsis and a couple of sample chapters. It is, however, essential to study individual publisher's requirements as these will vary.