JJ, I'm sorry to read of the problems with your neighbours. Certainly, some people, once upset, will become overly sensitive, and I suspect this is what has happened. You had one party, they could accept that, and they thought it was a one-off. Then you had another party, albeit not overly noisy to your ears, but YOU DIDN'T WARN THEM! Not warning someone of an event which is likely to disturb them, and more important, their sleeping children, is pretty bad manners, and not very good judgement. In my experience, very young children sleep through noise much better than older ones, like those your neighbours have. Admittedly, the language was uncalled-for, but angry people will sometimes lose their inhibitions!
We used to live next door, in a semi-detached house, to a family with two daughters, much the same age as our own. We got on fine most of the time, but the husband was a DIY enthusiast. He wasn't a bodger, but a very talented man - their house was beautiful. Unfortunately, instead of keeping his work on the house for weekends, he would work after he'd had his weekday evening meal. By then, it was getting late, and our girls were in bed. They, however, let their girls stay up considerably later, watching tv programmes which we'd never allow our girls to watch. Now, that was their choice, but keeping our girls awake wasn't on. I had a quiet word with the wife, after almost a week of it, and she understood, her husband didn't. It caused bad feeling for a while, but we eventually got over it.