This heinous act went down Friday night in Cape Girardeau, MO, a little after 11:30 PM, when two young kids were performing on a street corner ... busting some moves to music they had playing on a speaker, while filming themselves live from a nearby camera.

Soon enough, a man joins them and starts popping and locking too. His name is Micheal 'Crank' Curry, the one who posted the video. He tells TMZ these two kids are students of his -- he runs a local dance studio -- and they do this all the time without any issue.

The three of them were going back and forth for a while, until the taller of the two boys -- who's said to only be 12 years old -- got up to dance, only to get viciously attacked.

In the middle of his performance, a black CRV-type car pulls up alongside them, and a man hops out the passenger side. The kid notices him, but doesn't think anything of it as the guy dances along with them and inches even closer ... masking his true intentions.

When he's right next to the boy, he swings wildly and hits the side of his head, knocking him clean to the ground. Micheal screams out in shock and tries chasing him. The guy scurries back to the car and they take off ... Micheal comes back to attend to the kid.

Bystanders rush over quickly to help him up -- you can tell he's badly injured, but still tough as hell considering he was able to get up on his own two feet.

Micheal tells us the victim's name is Ethan, but he goes by Eman. The boy was taken to a local hospital, where Micheal says he was treated for a concussion and a bad nose bleed. Eman doesn't know who punched him or why -- it was a random act of violence.