YABU. I couldn't do it even if I could afford it.
I would be annoyed if I was sat next to a unaccompanied Dc whose parents were having fun in BC TBH. It is quite likely that they will need assitance of some sort at some point during the flight, and why should that be down to a random stranger? I would help as would DH, but my dad or FIL would refuse TBH!
My DC are incredibly well behaved but when they are tired or bored they do bicker. It's "don't touch me, don't breathe on me, stop talking, get over your ownside, turn your music down" etc etc. I would have thought that that sort of behaviour is pretty common, no matter what the OP says.
DH has to work his arse off for us to afford our European package holiday. Why should the likes of us have to sit next to the OP's DC because her DH is in denial about what they can and can not afford anymore?