Gallery and parents at fault here.
Gallery fault:-
If it's really worth that much they why not fence it off with a £20 bit of rope? If the art work is on loan then the owner has the right to feel furious it's being so casually displayed in a public area.
Parental fault:-
When your kid cocks up in public, the onus is on you to at least try and appear a little bit apologetic ffs!
Those parents come across as far more entitled and suffering from "worldrevolvesroundmeitis" than any I've seen on the Jeremy Kyle show - not cool!