If I owned a restaurant with a buffet, I wouldn't just have a stupid little sign saying kids under 12 have to be escorted, I'd totally ban all kids under 10 or so.
It's not just the problem with their grubby little hands handling the food at the buffet, it's the fact they get excited and start running around dropping ice cream everywhere when other customers may be carrying hot soup or coffee or something. I don't know how many times I've had to burn myself with hot soup to keep it from falling on some hyper little brat running around crashing into people. I walk with one hand on the soup bowl and the other cupped underneath it in case some little kid makes me spill it.
I'm an OTR truck driver, so I eat in restaurants by myself a lot. When I'm by myself it doesn't really bother me if some kid is standing on the bench seat next to me, staring at me, or if somebody's baby is screaming. But when I'm with my girlfriend or somebody else and we're trying to have a conversation it gets annoying.