Evolution: How We and All Living Things Came to Be by Daniel Loxton

Evolution: How We and All Living Things Came to Be is an excellent introductory book on the theory and explanatory power of evolution. Aimed a children, it is richly illustrated and manages to convey complex ideas in simple language. Additionally, Loxton addresses some of the common questions and confusions that surround the idea of evolution.

This would be an excellent beginner book for around 6-14 year olds (and some adults could learn something, too).
(Wired has a more comprehensive review)