It's a tough subject even for adults - I know I still struggle to wrap my head around aspects of blockchain. You'll probably catch their interest more with a storytelling style - the story of the 10,000 bitcoin pizza is an awesome myth, as is the anonymity of Satoshi. Get them hooked on the idea, and the ones who are feeling ambitious will be able to take advantage of resources like coindesk and ilk to learn more.

But I'm not a teacher, so I could be way off base.