Teaching them to have good routines is important. Washing and rinsing before bed is the most important tip I think. Sun protection and moisturising is another. I would use products specific for teenagers unless they develop acne etc.

It really depoends what thei skin is like.
For the very young use gentle products like Dermaveen or Cetaphil which is very kind to the skin and also cost effective.

Sukin is a great brand, and even has a teen skin range. Its easily available at grocery stores and regularly goes on sale. I’d try to avoid the trendy brands at that age, you don’t need to spend a crazy amount of money for products.