The Little Bubishi: A History of Karate for Children written by Andrew O'Brien is a book about the origins of karate, the masters, histories, etc. I have not read the book so I can't speak about it specifically but from the reviews that I have read it is very enjoyable for kids between 3rd and 7th grade.
The title comes from The Bubishi, which was originally a Chinese text which was passed from master to student and contains information about technique, philosophy, strategy, and good health. This is a book that the masters of our style of karate held in very high regard.