I witnessed child abuse, what can I do?

Today while I was shopping in the super market obnoxious, rude, "ghetto fabulous" woman came into the same isle as me with a baby, definitely under the age of 2, screaming his poor little head off while the mother tried calming him down by screaming even louder "shut the f*** up" and " wait till we get home I'll tear your a** apart". I could smell that the child clearly had a dirty diaper, which most like was why he was crying. This really made me mad so I ed in and said "I think he has a dirty diaper, if you change it he'll probably stop crying." Then she proceeds to start screaming at me "b**ch don't f***ing tell me what i should do with my child, I'll f*** you up". I just rolled my eyes and walked away while she screams "you betta f***in walk away nosey a** b**ch". So later when I'm getting in my car I see them coming out to the parking lot as well, child still crying. They were parked a few cars away from me so I could see everything they were doing. She basically threw him in the car and started slapping him like crazy, and I honestly didn't know what to do so I started looking for my phone to call 911 but before I got a chance the woman got in the car and sped off. At this point is there anything at all I can do? This is horrible and bothering me so much!