Introduction to Modern Cryptography by Katz and Lindell is from a pedagogical perspective one of the best-structured textbooks I've ever read on computer science topics (and in general, but my exposure to textbooks is a biased sample). It goes into the foundations of cryptographic theory and gives a good basis for understanding and contextualizing even advances that have been made since its publication. I think most children can grasp difficult material if an adult that's good at the meta-skill of teaching is helping them with it via individual attention