Maybe PocketBlock could help with that.
Check this site soon for more details. http://www.justintroutman.com/pocketblock/
In its current form, PocketBlock is a series of board-like grids that allows players to transform plaintext messages into secret ciphertext and convert it back again, one move at a time. By restricting the operations to little more than addition and subtraction performed by rearranging squares on a piece of paper, PocketBlock helps students understand the fundamentals of encryption without requiring a formal background in mathematics. At the same time, it stays true to the principles of modern cryptography and goes well beyond the classical cryptographic concepts, like the Caesar cipher, reserved for the most kid-centric material on cryptography today.