I've seen parents with a baby on LAX-LHR on the Upper Deck. We got chatting to them in the lounge and got concerned when we found out their seat numbers! Didn't hear a peep out of her on the flight. I've seen kids in F too - again very well behaved. The only time kids have annoyed me was on LHR-ATH on a 767, where a c.11/12 year old was a complete DYKWIA!