Senefer: A Young Genius in Old Egypt by Beatrice Lumpkin, for approximately ages 6 to 10 

Beatrice Lumpkin is one of the few well-known authors on African mathematics in antiquity. http://www.africahistory.net/lumpkin.htm
 
This really is a wonderfully rich picture book for Egyptian ancient history, culture, and math. The math moves from easy to some sophisticated ideas such as the Egyptian method of multiplication using the binary doubling sequence, so it has applicability to a very wide age range. Even as an adult I enjoyed it, the text is well-written, not dumbed down. Illustrations are excellent. One of the best math history picture books I've seen.

Review of Senefer: A Young Genius in Old Egypt by Beatrice Lumpkin