The point that Seth and some others are missing is that it's no more or no less acceptable for children to misbehave in FC than in coach. It's unpleasant for the parents and other passengers (and sometimes the kids) either way.

I don't like screaming children any more than anyone else, but life isn't perfect. And if the parent is legitimately in FC, then the lap child should be there, loud or not. First class passengers have no more or no less right to a screaming-kid-free flight than coach passengers.

Young children act the way they do because they do not know any better. It would be the same as if a FC passengers displeased about a screaming child started yelling at the parent and/or the child. The difference is that adults are capable of understanding things like etiquette, whereas many small children are not. Can you imagine what would happen in an adult were to do this?