Inconsistency is annoying to young readers and causes them to leave the story. Take time to ensure that the character quality is kept constant throughout the book. Even a character who is shy at the beginning can not just be outspoken.
The genre appropriate tone is, of course, the light fantasy novel or favorite comforting bed-time story as well. Consideration should be given to the scene and the time as well. Make sure there are no unexplained, abrupt time or location shifts that might mess up the readers.
To monitor such details, you might find it beneficial to have a style sheet containing character description details, preferred spellings, and themes to use when consistency is mandatory.