Dev Gualtieri's newly published Secret Codes & Number Games: Cryptographic Projects & Number Games for Children Ages 5-16 is a thoughtfully designed introduction to crypto for kids.
It builds from a potted history of secret messages to a series of simple substitution ciphers, DIY cipher-wheels, fun steganography examples (like encoding messages with flowers or cartoon pigs), to advanced subjects like keyed ciphers, the Polybius cipher, the Vigenère cipher, and some exciting work with vectors.
The $18 paperback isn't cheap (it's a Createspace publish on demand title, and they're pricier than mass-produced titles), but there's a real shortage of material like this, and the price won't break the bank. There are kids for whom this would be the perfect gift, and it would also make a great addition to library collections (especially school libraries). There's a preview here.
Secret Codes & Number Games: Cryptographic Projects & Number Games for Children Ages 5-16