Didn't look like assault to me... Looks like he was half-assing the drill, the coach came in aggressively to show that he wasn't taking care of the ball. The player gets knocked down, but if you watch the feet of the coach, it's clear he wasn't setting out to throw him to the floor-- the guy just went down hard and easily.
And the coach CERTAINLY didn't kick him when he was down. he nudged him with his foot when he just laid there, and said "Get up!"