Firstly, we're not in the UK, and here people to beat smack their kids. It is socially normal.
DD (17 months) had a bit of a breakdown in the chemists. She was tired and hungry. I was holding her and a few things at the checkout. She grabbed something and threw it on the floor and screamed. I was just about to pay, so couldn't really leave. She grabbed my neck and clawed me, then bit my shoulder. The shop assistant rang up the amout, then insisted on the correct change, which delayed things further, DD still screaming. She put the stuff in a bag, handed it to me, them smacked DD on the arm. Not hard, but deliberately. At this point she was screaming but not hurting or grabbing.
I didn't know what to do. I just glared at her and walked off. DD carried on screaming.
What should I have done? WIBU to walk off, or should I have said something?