Ex cabin crew here. We used to all hoik our judgey pants at parents who did this. Mostly from a safety/emergency perspective.
It’s all very well saying that plane crashes are rare, and that there’d be nothing you could do anyway, but actually there are all sorts of minor situations like go-arounds, very bad turbulence, bird strikes taking out an engine, overshooting of runways etc etc that can be quite frightening and aren’t always headline news so you don’t hear about them. I’d want to be with my kids personally. 12 is young to be dealing with something like that.
Aside from the safety aspect, I do think it’s a dick move. Fair enough these are teenagers, but I’ll never forget the four year old travelling in economy while her parents were up in Club. She was a confident, sweet kid, and very well behaved, but the man seated next to her was very uncomfortable with the situation. Poor guy ended up cutting up her food for her - rammed flight, crew were up against it just getting the meals out, mother was told to go and help but she faffed and delayed and complained and took ages to go to her.
The mother even huffed and puffed about being brought down from Club to help her go to the toilet. She wanted the crew to do it. 
This was an extreme example, but when I hear/see people putting their kids in economy while they waltz off up to Club, I’m always reminded of that mother.