I have a younger brother who would throw absolute fits if he died or something in a game. I spent a summer with him between college semesters and he noticed that I lose it with laughter each time I'm really trying in a game and fail (laughter intensity tending to be proportional to how hard I'm trying). I noticed that it started to rub off on him, so I made greater efforts to point out how hilarious I found whatever was happening. I think he still has trouble, but for the most part he snaps out of it if he is reminded. Not sure if that is possible with whatever ages you are dealing with, but good luck!