1-2 is near impossible but by 3 they should know better. Assuming (Ha!) the parents instilled discipline from the get-go.
My Mom and Dad raised eight well-disciplined kids, and Mom always said that the terrible twos were a myth - it was the terrible two-and-a-half and three that would get you.
Granted, there's a difference between a kid who's almost four, and we don't know how old this little girl was. In either case, even if they couldn't shut her up instantly, they could have moved her. That's the part I don't get. Same with slapping Mom. That wouldn't have flown - no pun intended.