The National Eye Institute is a division of the U.S. National Institutes of Health, and they are offering two free activity books for kids!

See All You Can See: Activity Book for Ages 6 to 8

Wild About Healthy Vision: Activity Book for Ages 9 to 12

They contain  pages of word puzzles, family surveys, projects and experiments, and more designed to help children learn about parts of the eye and how they work, as well as eye safety and first aid for eye injuries.

(I assume they will only ship to a US address)