[Rachel Bronwyn]

There is one sufficiently large target in a fluid situation to shoot at where the likelihood of accomplishing rapid cessation of actions is reasonable: "centre mass", in the upper torso. Anything else is fantasy cop show stuff.

That being said, the reason you shoot someone is to prevent them from committing further harm. Dude's going to have to prove the restrained kid was committing or even capable of committing harm whilst restrained to justify shooting him.