Not every children's book uses pictures--they're more prevalent in books for younger children than older children--but when they do, you need to plan for them carefully. Chart the number of pages you want (you may need an editor's input if you intend to publish) and how many of them require pictures. Then sync up the text so that you know which passage goes on each page. You can then determine what kind of picture should go on each page to match the accompanying text . Work with the artist to make sure you capture all of the imagery you want. You may also need to alter the text slightly to better fit the layout--ensuring that every page with a picture contains some good prose with a lot of visual potential.
Books which appeal to a 5-year-old are apt to be boring to a sixth grader, while sophisticated works aimed at bright elementary schoolers will go right over a toddler's head. Prepare for your book by studying the psychology of different age demographics and determining which one is best suited for the tone you have in mind. Speak to local teachers and see if they can offer you any advice, or look for books that stress how to communicate with children of a particular age. Then write to your chosen age group, using words they will grasp easily and situations which resonate with them.
Many children's books allow for an element of the fantastic--talking animals, magical lands and extraordinary events taking place throughout the narrative. Other children's books focus on social issues and other important topics. Whatever your story line, it should move quickly, contain plenty of action and avoid long passages of dialogue. You also want to stay away from cliches. Don't just regurgitate popular fairy tales in new forms and stay away from chestnuts like magical artifacts or quests to save the world. Look for the creative, the imaginative and the new: something no one else has done before. Your book will improve as a result.