Another book that is fun is You Can Count on Monsters, which is "about" prime factorization, but is a picture book of 100 monsters, each of which has elements based on its prime factors. Lots of fun little details to pore over while getting familiar with numbers. Games are also a great choice. Prime Climb is fun, and Incan Gold is a great one for subtly teaching division and remainders (and is fun for all involved).