I personally think they should have either left the baby home with someone they trusted or elected to not attend this particular party. I think sometimes "stars" think that basic rules of etiquette don't apply to them, just to everyone else. The baby's name was not on the invitation, she shouldn't have been there.
Having said that though, stop and think of the reasons most people don't want kids at a specific 'adult' event. It's usually because the kid cries, throws tantrums, gets into things, breaks stuff, demands attention from other party goers, etc. This baby was so young that she would be guilty of none of those things.
If she fell asleep early in the game, it is possible/probable that a lot of the partiers weren't even aware of her presence.