Know your audience. The readers for chapter books are usually between the ages of 6 and 9. Read books for children in this age range to get an idea about the subject matter that appeals to these children and the length of the book that you will write.
Set a limit on the number of pages that you will write. The average chapter book is 40 to 70 pages. You need to keep a new reader's attention, which is difficult with a long story. Use short sentences and words. Short words and sentences keep the reader focused. Think of chapter books as guides that nudge a child along her reading journey.
Do not use a lot of adjectives. Keep your descriptions short and choose your words carefully to distinguish your characters and plot. Use words familiar to children, but stretch vocabulary with only a few new words. Tell a story without embellishment or fancy rhetorical tricks. Use the rhythm and words of natural language in your writing.
Create books with straightforward time sequences. Going back and forth in time during a narrative unnecessarily jumbles the story. Linear continuity in chapter books keeps narrative simplicity and allows you to write a short, uncomplicated book.
Keep your characters to a few because too many characters can clutter stories. Choose to create characters that children can identify with, like children their own age. Put your characters, whether they are human or animal characters, in ordinary situations that children can understand, such as the first day of school in a new town or being away from home alone at overnight camp.
Do not lecture in your books. Remember you are inviting children to be life-long readers with your stories, not preaching at them. Create a story that speaks to their experiences as children. "Alexander and the Terrible, Horrible, No Good, Very Bad Day" by Judith Viorst, for example, is a book that children can identify with because it touches on children's feelings about having days where everything seems to go wrong.