Mothers and fathers can generally treat their various children in a similar ways and one could even say equally, but they are not going to feel the same about each of them.
When a child is in a new and unknown situation that is emotionally charged it will try and read how mother is emotionally reacting in its attempt to get a base line for itself.
I am not arguing for or against laws about smacking of children. What I am saying is that at some point it has to stop. Using the law to take away more power from the parents simply leaves a child feeling more insecure.