If your kids get into coding and if they're doing Javascript (which I think is a good place to start for lots of reasons, but python might be better) I can personally recommend Free Code Camp for learning basics and intermediate topics in javascript web development. The interface is thoroughly gamified and it's very effective for people who are, but ONLY after they're enthusiastically hacking away at web dev. They'll be much more successful at learning the basics once they realize that they want and need the basics.