What to do about spanking threat at mall?

Was walking thru the mall last weekend. Family in front me: Mom, Dad, 2 boys maybe 7 and 8 years old. Boys were doing everything possible to get on parents nerves: screaming, stomping, pushing and tripping each other. The dad was ignoring most of it. The mom was clearly agitated. She jabbed the dad with her elbow. He took off his belt - a real thick leather work belt - doubled it over and slapped it against his thighs. Made really loud crack. Got kids attention. They stopped horsing around, but the dad kept walking behind them with doubled belt in his hand, but he never actually spanked them with it. Nobody else in mall seemed to pay attention when he slapped the belt. I just happened to be going in the same direction. We walked out the same door but went different ways in the parking lot, but the dad was still carrying the belt when they walked out. Should I have followed them or said something, or was I right to mind my own business? PS: I'm in my early 20s, not married and don't have kids.