In the last scene, at Sonny's surprise birthday party at Hooters, his wife is holding a real baby that he kisses, etc.
Check out "the baby" as they move toward the birthday cake, not only is it obviously fake, but it shrunk about 6 months and was stiff and covered up and she was holding it like a football!
Almost all movies use shots of fake dolls. Remember that children can only 'work' for a short period of time before they have to have a break. I remember watching something on the Olsen twins. When they played Michelle on Full House one twin would work for 30 minutes then they used the other twin for 30 minutes.
I agree, this is no slip-up. It's just the manufacturers of "fake-baby-doll-props-for-use-in-movies" really need to start making more realistic, heavier, less-stiff dolls :)
Note, the prior comments. Understood that baby's are used on a limited basis. But I believe it is a slip-up when the realism is lost. It's like using a stunt double that doesn't look physically like the star. Tough crowd!
The title of your "slip-up" should be: Daddy Little Boy. The baby is in a blue blanket, so I think it is a boy. Just wanted to clear that up. Also I'm going to have to agree with the other 2 comments that this does not fit the definition of a true slip-up. Good eye though!