I found a book though I haven't read it so I can't say how good it is. Great review in Cannabis Culture cut and pasted for her pleasure.

This is a book whose time has come, a book to help pot-smoking parents explain marijuanna and prohibition to their kids. Beautifully illustrated and clearly written, this book introduces children to the medical and recreational uses of pot, outlines the plant's botany and uses, and explains the history of prohibition and how activists are working to change the law.
The story begins with little Jackie waking up past her bedtime, and smelling "something funny" in the air. She finds her parents smoking a joint, and so the next day Jackie's mom takes her on a bike ride to discuss marijuana. They visit Farmer Bob who grows organic buds, Dr. Eden who explains medicinal use, and some police officers who describe the state of the laws. The story ends with Jackie declaring "When I grow up, I am going to vote so I can make all the laws fair."

www.justaplant.com