YANBU
I can't bear parents who think that you shouldn't be allowed to so much as look the wrong way at their children, no matter what they do.
I had this twice at soft play the other day. First with three older children who shoved in front of me in the cafe queue - literally pushed me out of the way - I said "Er, excuse me," pointedly and they apologised meekly and moved, but then one of them went over to their mum and was clearly complaining about me, leading to much glaring from the mum. Second incident was when I refused to open the gate into the baby area (I was sitting right next to it) for a child of about 6 who got a bit stroppy. I told her to leave the gate alone and go play somewhere else - again, straight over to mum who glared and made obvious comments about me to the people she was with - and got a bland smile in response.
I also had a minor run-in with a mum at a farm park a few weeks ago. I was standing with the DSs and their cousin while their gran took the other cousin to the toilet. There was a coin-operated ride nearby with another child of about 3 playing on it. They went over and stood nearby. I told them not to try to get on as another child was there, and that we were leaving in a minute. DS1 touched one of the cartoon figures on the ride, but made no attempt to climb on, but the little boy on the ride kicked him in the head. Not a hard kick, but a firm one if you know what I mean. I said "Don't do that please." He did it again, so I pulled DS1 away and said "No kicking." He then disolved into tears and started screaming for his mum who walked over to see what was wrong.
I explained what had happened and she got hugely stroppy with me and said "Well it must have been provoked." I said it hadn't been, but DS1 was fine and these things happen,etc etc, all very conciliatory. She insisted on questioning her DS about what had happened, all very much geared up to my DS1 being in the wrong. "Did he do something? Did he hit you? Were you frightened?" with me standing there saying "Really, he just kicked him. But it's fine, we'll leave you to it. He's fine." She was quite snappy with me, saying "well if you say he kicked him then he's going to have to apologise." More screaming from her DS.
We started to walk away and she came after us and insisted that he had to apologise, which he refused to do, so we all stood there like mugs while her DS screamed and rolled on the floor. I kept saying we should leave it and that DS1 would have forgotten about it in 5 minutes, but she was getting really wound up and saying "No, he's got to apologise" and somehow none of us could muster the gumption to say "this is daft, we're going." He eventually apologised, at which point she glared at me, shook her head and stomped off. Clearly I had somehow angered her by my child being on the receiving end of her child's bad behaviour!