Flash is a known attack vector. I'd be more comfortable with the argument "Kids should be free to play games" if I didn't have to keep purging virii off my aunt's computer because she won't stop visiting Flash game sites.

Besides, there are alternative browsers. If kids need to drop into Edge to play Flash games, that seems like an acceptable solution? If it isn't possible (because they're on a school network that whitelists browsers, for example), that's indicative of the cost / benefit tradeoff of that use case.