Definitely not limited to children, I observe this in myself too. (although hopefully I handle it better than a child)
I suspect that is because when playing a physical sport, the physical exertion gives you a dopamine rush whether you lose or win. When playing video games, you have no such release if you lose - just pent up frustration.
I don't think that describes feeling and behavior after people playing sport. Not in my experience at all. It is other way round - angry sad people are happier after physical activity (which does not have to be competitive anyway).
I suspect that is because when playing a physical sport, the physical exertion gives you a dopamine rush whether you lose or win. When playing video games, you have no such release if you lose - just pent up frustration.