For a 10 year old I would highly recommend Martin Gardner’s “Codes, Ciphers, and Secret Writing”: https://a.co/d/05rZPKiH

It’s a slim volume that focuses on historical and fictional ciphers. It stars with simple shift ciphers like Caesar’s cipher, progresses to more complex substitution ciphers with examples from Sherlock Holmes and Edgar Allen Poe, then covers polyalphabetic ciphers like Vigenère and Playfair. It covers basic cryptanalysis and has practice riddles and cryptograms throughout. It even has a chapter on a variety of ways to make invisible ink, which is often an object of fascination for children around that age!