I was renowned, pre kids, for having the longest fuse of anyone. Then I had kids. Once, a year or so ago, I was in a very small, enclosed playground. This dad came in with his 12 year old son or son, plus about 4 little kids. The little kids started running around, swearing at each other, one was calling the other a fucking bitch, they were yanking on each others hair, kicking each other, shoving each other into the dirt, swearing some more etc, meanwhile the dad, who was right next to them letting them hit each other and call each other bitch etc, was hoofing a fucking football around the playground with his oldest son, it was ricocheting off all the wooden structures and as it was such a small playground, it was smacking into other kids, smacked mine 3 fucking times, they never said sorry. Just hoofing the fucking football around whilst his feral children ran amok.
My 3 came over to me and I said, look, just stay near me, they've just said they're leaving and once they've gone, we can relax. Other people also left the playground because of them.
My oldest, who was then about 10, said one of the little girls was threatening my 6 year old daughter, saying she was going to throw her off the top of the play castle etc, kick her head in. It was insane. The family then go to leave when one of his 6 year olds turned around, stared at me, then marched up to me with a grin on her face and said "he, HE, called me a BITCH". My ds looked horrified - and my other two kids instantly said, mum, he didn't, it was HER swearing, so without looking at her because I was so angry by that point, I just held up my hand and firmly said "No, thank you. Please leave" (I worked then in a nursery and with bad behaviour we were always told to say no thank you). She stared at me. By that point the dad had left the playground, but had stopped with his back to us, obviously listening in. But not doing anything. The little girl then repeated it louder as if she couldn't believe I wasn't shouting at my ds, pointing at him and stomping her foot. "HE, HE CALLED ME A BIIIIIITCH!!!". And I couldn't help it, without looking at her, without pointing etc, I just held my hand up again and just said "NO". And that's when the dad spun around, "You don't tell other people's kids no! You don't tell other people's kids no!" etc etc. So I lost it on him. I pointed out he was hoofing that stupid ball around, how it had hit mine and other people's kids, how his kids were smacking each other and swearing and how his kid had threatened to throw mine off the top of the tower. He just kept repeating himself. So I ended it with "well if you parented your own children other people WOULDN'T HAVE TO!".
It's the only time I've ever truly lost it with someone. My mouth was just talking, and I was just so angry, and it was like I was watching myself shout at the guy out of body. I was ashamed after. But my kids still talk about "the demon child and her demon dad" and they can laugh about it now, but at the time the other kids really scared them.