should you and your kid speak french or spanish, "la bible des codes secrets" by Hervé Lehning, the bible of secret codes, is a book I greatly enjoyed as a young teen. it walks you through the history of cryptography until the current times, and has a lot of examples where you can try it out yourself. it is almost not coding-oriented, with only the last 3 chapters explaining modern-day encryption, but as far as my experience goes, few ten-year old enjoy reading documentation or textbook-style books. This one is very didactic, and also an interestin tour through history.