I daresay she probably didn't notice that her kids had started it. Probably an honest mistake. I'm sure we're all guilty of not observing 100% when at a park.

As for the reaction, I don't think yelling is parenting. I think it's losing control. I'm not saying I haven't yelled, just that I don't think it's a finer parenting moment. And if someone else yelled at my child I would probably parent them on my views on yelling in a calm way. I'd also expect my child to apologise (I don't believe in tit for tat, although sometimes it's hard not to wish I did!). But I wouldn't have allowed my child to be treated that way. And I would have told my child the same thing that he needs to say stop. And if that didn't work I would end up leaving.

I'm a crazy person, I believe it takes a village to raise a child. But I also think there's room for misunderstandings. And I think sometimes we get caught up in the mama bear stance that we forget they are only children, and the other parents are learning just as we are. And judging them for their child's behaviour or worse judging their child for their behaviour only makes for a bigger divide in our "village". Calm and resolution (but also standing up for oneself in a non aggressive way) are all equally important and good examples to set for our children.

Perhaps Mother B was shy or had a language barrier. Or maybe she was having a bad day, and was absorbed in her own thoughts. But I don't think it was neglectful or anything like that.