Having a son has made me think about how we can introduce nonviolent action to children in terms they understand. I have found very few books that help.
However, one book I particularly like is a book called Click, Clack, Moo. It is a story about cows that learn to type and start making requests of Farmer Brown. When Farmer Brown refuses to implement their requests, the cows go on strike. The PBS Show Between the Lions, which is a great show that focuses on reading, has a version of it on-line. The teaching unit is also on-line.
Here is more info about the book so you can find your own copy:
CLICK, CLACK, MOO: Cows That Type
by Doreen Cronin
Illustrated by Betsy Lewin
Simon & Schuster