- A central character (protaganist) who is the age of the intended audience.
- Central issues of the story concern children in some ways.
- A straight forward story line.
- A linear and limited time sequence in a confined setting.
- Language is concrete and vivid and not overly complex.
Not too sure if the last 3 points really apply anymore. Children's book is getting more and more sophisticated. Even the age of the protagonist may not be a good gauge now.
However, the 5 points above do serve as a guide. : )