How do you all deal with other people's violent children?

My wife and I are really close with another couple, we're all in our 20s. We both have a 3 year old. We spend a lot of time with them and get along super well, they are like family to us. Our daughter is super sweet, kind, never takes other kids toys, etc etc. Their daughter is a really sweet girl too, very funny, but she also can get very violent. It started off with their daughter randomly pushing ours, but over the past few months it has escalated- punching, grabbing hair and pulling her down to the ground, kicking, taking toys out of her hand and smacking her in the face. These instances happen about every 5-10 minutes for the entirety that they play together. When it's on the more violent end of the spectrum, her parents will either spank or put her in timeout. When it's "lighter" stuff, like stealing toys, or disregarding pre-established personal space like "you two are not allowed to touch each other", her parents don't usually do anything.My wife and I have talked about this to death, and here's where we are torn:On the one hand, I think it's good sometimes for kids to get roughed up. I would never want it to happen, but when it does, it's a good opportunity for them to learn that people will wrong you, and it's up to you to decide how you're going to respond. Things don't always go your way, and I'm not going to raise a kid in a sterile environment and try to constantly insulate them from the world.On the other hand, if someone at work was constantly coming up to me and grabbing my hair or punching me, if after I've told them to stop, and I've told my boss and he didn't do anything about it, I would take it into my own hands and knock the shit out of them. In our modern society as adults, it's appropriate to go to the authorities (boss, HR, police, courts, etc) and report things, but if they don't do anything about it, you have to defend yourself.So as her parents, we've already stepped into the situation a few times and disciplined their kid right in front of them when they weren't doing anything about it (not physically, just time-outs or separation). But our kid has started to literally cry when she finds out this kid is coming over, and I don't blame her really, because she knows her ass is about to get kicked for 3 hours. Our kid is not one to fight back, she couldn't hurt a flea.What should we do? Do we just let it keep happening and intervene when it does? At what point do we talk to our friends about this? They are fully aware that their kid is violent, and we don't blame them, they are good parents and try to do the best they can. But at some point we're like, this needs to stop! via /r/Parenting https://ift.tt/2XxCvlo