Hey, I just got my new issue of Family Fun today (where I first found ATT), and they have a section about great code books for kids. There is a deck of cards with codes, anagrams, puzzles, etc. called Mensa Secret Codes for Kids (age 7+), the Simon Singh book (12+), and another one called Top Secret: A Handbook of Codes, Ciphers, and Secret Writing (9+). If your kids are working on this with you, and you're having trouble explaining some things, these might help, and they sound like fun! There is also a mystery novel involving codes called Chasing Vermeer (8-12).