If I was the kid parents I'll pressed charges on the guy to pay the dentist bill, no more and because You don't hit MY Child.
However I'd say to my child that even if the over-react, it's that could happen when you annoy someone, ON PURPOSE. You never know with whom you're dealing with. Without being paranoïd, it's not only kinder but mostly way safer for yourself to be "kind" with other people.
I live in a town with a psychiatrict hospital nearby. Some of the less "crazy" patients can go into the town and have a somewhat normal life. But as a kid you learn very soon that while they're are harmless (even if Strange in a kid POV) some are prone to get angered by the smallest things (they scream a lot, in 20 years they're only one case to physical violence toward a kid, and, the kid provoked the man calling him names because "It's funny when he's angry". So you learnt to behave in public very very soon
Let's hope the kid learnt a lesson here and will stop his bad behavior.

Or, she's a 3 year old and like almost every 3 years old on this planet, she enjoys and had fun seeing herself in the TV. Also I can't believe someone could play Doctor and try to diagnostic a little girl while seeing her 2 minutes on TV. It's like saying "I saw someone cough in the bus today. Base on his skin complexion at the time and the way he stood He may have Lung cancer".