First, you must understand the genre. Genre is a French word referring to a category. Fiction and nonfiction consist of many genres (fantasy, science fiction, biography, memoir etc). Within the children's book category, there are several types of books.
Picture Books are divided by age groups and are frequently musical (rhythmic), succinct, and pictorial since you are using illustrations at this age to transmit part of the meaning in your story. You don’t necessarily have to use rhyme, especially if you are not good at writing verse. Nor do you have to limit the vocabulary. Write the story and allow children to inquire about the meaning of words to broaden their minds. Besides, the publisher/editor will request changes if they are deemed necessary.
Here are the age groups:
Toddler/preschool books (up to age 3) have stories that are basic, simple and short (up to 300 words with a 12 page average) containing brightly colored illustrations that are often about colors and shapes. These books generally have no plot and the illustrations are a key feature in telling the story. However, these particular aspects may vary with each publisher.
Nursery School/Kindergarten books (3 to 5 years of age) also have many illustrations and include short text about familiar objects from their world. These stories have more rhyme and introduce simple plots about hopes and dreams or resolving problems.
Beginning Readers (6 to 8 years of age) are drawn to picture books that show good character development and action. These books have interesting story lines with humor, and help the reader explore the world. Chapter Books become important at this age as children enjoy the stage where they are beginning to read by themselves. You can easily create small chapters including sentences that are short and words that are simple but are also in line with this learning level. These books average between 32-64 pages in length and contain up to 2000 words.
As children grow and learn, so should their authors. Therefore, the stories become more complex and the plot thickens. Authors must learn to introduce new and interesting characters that they know inside and out, decide what message they want to share in the story, and learn to develop a plot.
The goal of writing stories is to introduce the characters, explain the conflict and close with a resolution. The formula of 25/50/25 may be helpful to you at this point; 25 percent of the pages revolve around character development near the beginning of story, 50 percent of the pages involve the conflict and the final 25 percent illustrate the resolution.
The next phase of books for children may be more challenging to write:
Transition books (7 to 9 years of age) or early chapter books bridge the gap between beginning readers and chapter book readers and are about 30 pages long, broken into 2-3 page chapters.
Chapter books (7 to 10 years of age) are approximately 45-60 manuscript pages long. Chapters are now divided into 3 to 4 pages, with more complex sentences, short paragraphs and stories that are more substantial and still contain action.
Middle Grade books (8 – 12 years of age) are geared towards a prime reading age for most children. Here you can introduce secondary characters and more complex stories with sub-plots and fast paced action. Humor is very popular at this age. They average 100-150 pages and begin to open the range in genres.
Young Adult books are suitable for ages 12 and up and average 130 to 200 pages with complex plots involving several major characters although the main focus is usually on one character. There are fiction and nonfiction books in this category that deal with life and the reality of everyday struggles.
Having broken down the types of books for children, look for part two of this article that concentrates on the basics of getting the story from your head to your paper.