I realize the plane can't move until everyone is in their seats. It just seems they could have let the parents walk around holding the child for a few minutes to see if the kid calmed down.
Well we weren't there and probably don't know that the airline in fact didn't give them some time to calm the child. From the sound of the story I would postulate that the child was totally out of control.
They evidently couldn't do that. They couldn't manage to get her off the floor.