Children in this regard are no different than any other passenger: maybe they behave themselves, maybe they are obnoxious. I have shared the F cabin with several obnoxious kids (which truly is the fault of the parent). I have shared the F cabin with several obnoxious adults. And I think the number of obnoxious adults I've encountered is far more than the number of obnoxious kids.

Personally I am not a "child friendly" adult but if someone is willing to use their money, miles, or benefit of having a companion on the same reservation to upgrade their kid(s) I don't see any reason to stop them. And while a young'un sitting in F will add a bit of extra annoyance if I'm sitting in Y, I'm still going to be annoyed to not have been upgraded regardless of who's in the F cabin.

Stuff like this used to really irritate me but it's one of those things that is out of my control and therefore not worth me brooding over.