I'm generally a pretty accommodating person, but I agree with what others have said that it isn't worth stressing too much about. Like you said, they were the ones who choose to live on the bottom level knowing there would be people above them. It's one thing if you are having loud parties at 2 am, but to tell kids not to run around in their own home? Not going to happen. As suggested, maybe if the kids are being exceptionally wild, remind them that everyone has to be considerate of each other and that includes not making too much noise for the neighbors, but otherwise let 'em go.