If I was travelling with a child in Upper Class, I would make them very aware that it was "very special" and that "kids were only allowed there when on their best behaviour" (not exactly true, but makes them feel special and understand that good behaviour is important). This is what my parents did when they took me to a posh/special restaurant when I was a child - seemd to work pretty well... I would probably also make a point of showing the child som eeconomy seats - to point out how lucky they were (and how importantgood behaviour is).
Boo