I've flown alone with a young child, and while I've tried to make her behave, it's not easy.
I'm she would have been one of the children you are talking about. She is an incredibly active child, who also becomes hyper reacts very badly when she has sugar.
I packed a selection of healthy snacks and food, but unknown to me, dd was fed sugary treats by other parents, who didn't ask me first.
She wasn't being 'naughty' as such, but rather noisy (squealing and giggling etc). I did remind her repeatedly to keep the noise down.
Another passenger flipped out, saying my child had adhd and I was a bad parent.
Flying with children is not easy, especially for medium to long haul flights. I wish there was a little more understanding of this. By all means parents should try to control their children, but not many under fives will sit still and stay quiet for over 4 hours.
Perhaps there should be child free cabins for people who are unable to understand this.