Identify the age range you want to write for. Publishers--as well as booksellers--strictly define the youth market based on vocabulary, age-appropriate themes and cognitive development. Accordingly, your book must clearly fit into one of the following categories: ages 5-8, 9-12 or 13-18. For children younger than 5, material revolves around picture books and simple stories read aloud by adults.
Decide whether your book's content will primarily be educational, entertaining or inspirational, and determine the best genre to deliver that objective. Examples of genres include comedy, fantasy, science fiction, western, drama, mystery and historical. In choosing an underlying theme, consider the issues, dreams and fears that will most strongly resonate with your target demographic. Examples include security, friendship, courage, acceptance, happiness and adventure.
Construct a plot that's commensurate with the age and abilities of your lead character. This character--who will be at the center of all the action--should be the same age or a few years older than your target readers. The older the reader, the longer a book can be and the more complex the problem that requires solving. For example, a mystery for third-graders about a missing cat would run between 40 and 50 manuscript pages. In contrast, a mystery for teens involving an archaeological expedition, an estranged father and a first crush would average 200 to 300 manuscript pages.
Decide on a setting and time period. Although young readers enjoy and relate to backdrops that are familiar to them--home, school, church, playgrounds--they also like learning about distant lands and earlier centuries. As long as your content is factually accurate, fictional stories juxtaposed with real-life historical events are an effective way to tweak their curiosity and make them want to learn more.
Create a beginning/middle/end outline that equitably parses out the action and progressively escalates the stakes. An easy way to create this outline is to divide your projected page count/word count by three; this tells you approximately how much space to allocate to each section. Your young protagonist's conflict should be apparent in the first chapter, and all of the main players--including the villain--should appear no later than the third.
Ensure that your protagonist maintains a moral center and exhibits positive behaviors your readers will admire and want to emulate. Create conditions under which this character can take chances, learn from mistakes, consider the well being of others, accept her imperfections and take responsibility for her actions.
Subscribe to publications such as "Children's Writer Newsletter" to stay abreast of market trends, as well as read interviews with agents and editors open to new submissions. This entity also publishes an annual guidebook and offers online writing courses. Study the submission requirements in annual resources such as "Children's Writer's & Illustrator's Market" published by Writer's Digest Books. This directory not only identifies book publishers that accept unagented submissions, but provides contact information for literary agents and professional competitions.