<human>: Can you condense the text into a few key points?

I was in the cheese isle of my local grocery store.  There was an elderly couple most likely in their 70's as well as a few kids and their grandmother.  
The kids were playing and laughing but nothing too obnoxious when all of a sudden their (presumed) grandmother runs up and whips the little boy (about 4 or 5 years of age) across the back as she screams SHUT UP!  As any young child would do he fell to the ground and began crying. She continued to whip him very hard with the leather belt as he curled into the fetal position. I was stunned by the ferocity of the beating so my reaction was slow but after the 4th strike I stepped between the lady and the child.  I picked up an item off the shelf and pretended to continue shopping.
At this point the lady angrily told me to "move mother fucker" and raised the belt.  I pretended to continue shopping and she said the boys name, he whimpered but got off the ground and followed her.

What do you do in that situation?  If that woman is willing to beat her grandchild that hard with a leather belt in public I am worried what would happen behind closed doors.  I did not call the police but feel some what guilty for not doing so.  What is the proper procedure when dealing with this?
<bot>: Saw a lady viciously beat a child with a leather belt, stood between them with out saying a word until they left. What should you do?