Crypto (cryptology) is an interesting field that offers interesting ways to learn math. Both cryptography or cryptanalysis require strong analytical and creative thinking that strictly roots to math.
After achieving some coding/programming skills level, kids and be introduced to crypto to challenge their skill in problem solving. In cryptography they will learn how to hide (encrypt/encode) messages and revert it back (decrypt/decode) it to the original text. This way they will learn about symbols, correlations, associations and causality. Skills that will strongly escalates their math skills.
While in cryptanalysis, kids will learn how to solve problems systematically, with several different ways. This will encourage them to be creative by thinking about alternative solutions by analyzing probabilities. Cryptanalysis skill will gives kids ability to adapt to different situations and problems and find different ways to solve them.
The other most important thing is that crypto is a field that everyone, not just kids should have knowledge about. Because one of the consequences of living in this digital world is to be aware of security aspects of accessing and exchanging data. That makes crypto literacy, or security literacy is important to everyone.
Code - and rule the world 