I'm not a parent (I don't think :shock

so leaving aside the question of appropriateness of 14yo chaperoning 2yo (though it was a <1 hour flight, not exactly CHC - OSL) I'd agree that the website is misleading.
The lady booked 1 x Adult and 1 x Child(2-11). It is disingenuous of them to say...
"A 14-year-old is an unaccompanied minor and they can't accompany another unaccompanied minor.
"It's all listed on the website."
...when the 14yo was clearly booked, within QF's own rules, as an adult.
If they consider a seat worth the same whether a 14yo or a 24yo is sitting in it and want to charge a 14yo as an adult then fair enough, they're occupying the same seatspace, but do not then try and have it both ways by redefining an adult for purposes other than fare elsewhere on the site.
I have had little to beef about with QF (FF enhancements excepted, and after expressing your disappointment you just learn to live with those and use whatever's left to your own best advantage, or move schemes) but this is one case where they need to be treated as a minor (accompanied or otherwise), and given a good smack!
PS. I don't know whether smacking is considered acceptable these days for minors...if not then QF should be sent to bed without dinner, not be allowed to watch Romper Room, or whatever punishment
is PC today.