There are some people whose idea of compromise is when the other party does what they say. Note: that was something explicitly said in USA politics at some point, but I honestly see it on both "sides", and surely happens in many other places in the world as well. I've seen parents literally lift up urine-soaked & still dripping children from non-plastic seats, only to either not leave a tip at all or like a bare minimum 10%, while leaving ~50-fold more mess spread out everywhere than any other table in the place (and not even tell the servers about the urine, which would need special cleansers to not become embedded into the furniture). My point is that it is not the children - the innocents - it is the parents, who are inflicting their choices onto you. And I suppose the owner too, but they are customers and what, are they supposed to just turn them away? Like gentrification, they come in and you can gtfo b/c they own the place now (childification?:-P)
Oh wow, it just struck me how much your story parallels what happened on Reddit too... aside from spez's antics, I likely would have left it anyway b/c all the children were taking over there too (they definitely are in charge of so many Discord servers). You can fight back (talk to the owner), or you can leave (maybe there is a place to get a haircut inside of a place of employment, where children are less likely to visit? or perhaps if you travel further away?), but definitely pick your battles b/c the kids, and more importantly their parents, aren't going to disappear anytime soon - if anything, the sort that you are talking about seem to be getting bolder all the time.:-( Maybe this is why people go to country clubs, to filter them out?