Hate parents like this, there seems to be loads of them too.
Not quite the same thing but similar. When DD was maybe 2, we took her to softplay. This lad, about 7 ish kept pushing her over. Parents just sat there beaming. It conitnued so I told staff, they basically said the woman had paid for their child the same as I had 
I kept moving DD away from him but he would follow her. Mum still saying nothing. I eventually asked her is she was going to control her child and she gave me a mouthful about how it was softplay and DD wouldn't actually get hurt. Then went back to just beaming as he crashed into numerous kids on purpose, when he was too old to be in that area wnyway! (was for under 5s) He then BIT her on the shoulder. So I told him off. And suddenly she could get off her arse and storm over to tell me 'noone tells my child off but me'. I told her if she had sorted it then I wouldn't have to be involved as..he wouldn't have done it. She asked if I was calling her a bad mother and I said basically yes. Eventually staff chucked her out as she started on another mum after the mum also had to tell the kid off after he punched her child in the face. She swung for this woman though, I suspect she didn't do that to me as I was a lot taller, and a lot lager in general than this woman.
Some parents are just really really shit. I don't care if this sounds judgemental or not., Learn to control your kids, or don't take them out.
DD has Autism also (we obviously did not know when she was 2 though, and it changes nothing as she did nothing wrong), so I don't really take that as an excuse. Yes, it can make it harder for her to behave, but if she was assaulting children, or ruining something completely for others (as in the OP) I would remove her. Its easy enough done. Yeah sometimes it seems harsh as often she cannot control her behaviour, but there is no reason at all for everyone else to be disadvantaged, simply ass my child is Autistic. Definitely no reason to condone violence either or just sit laughing while she pushes kids over.