Our experience at Fortnum & Mason wasn’t what i’d describe as “quiet elegance”. It was classy, but there was a piano player playing a piano and a kid’s birthday party going on at a large round table near us. The children (8-10 yr olds, boys and girls) behaved well, but there was lots of talking and laughter from them and many other tables in the room (which is large, spacious, airy, and lots of windows), piano music, and many servers moving all over. It was not a stuffy, nose in the air atmosphere. But yes, i wouldn’t bring a kid there (or anywhere) that can’t behave appropriately. I think it’s helpful to give jleonardi many recommendations and they can pick which kind of experience they’re looking for.