I think it reminds me of when I was young and would spend days doing sets.
Do you want to do sets with them?
Not particularly, I'm just wondering why they are playing that.
Even children work hard. Knowing that their hard work is appreciated would make them feel good and work even harder.
I agree, but what do you suggest should be done to improve?
Maybe reward them for their hard work? Let them know that you're aware of the effort they've put forth so that they can feel pride and work even more. They'll grow up with a better worth ethic as well.