I'd like to add one other point.

It takes a lot of thought to be good at a game as well as good reflexes.

I think its better to play one very long stretch, say 2-6 hours, then not go back for a week, while thinking about my mistakes and the flow of the game and the scenes.

If a kid just plays it, then he or she is not going to get better. Its just going through the motions. The mind and body need time to rest and recover.

If you continue to allow them to play, perhaps posing the question, "how to get better more efficiently?" and "Do you enjoy the game more if you take a week off or just play it constantly?"