Regarding the 7th grader taking out the center, and you had suspicions he might have gotten away with something. "Fool me once, shame on you- fool me twice, shame on me". Level is really not significant, as this maneuver is as old as the game itself.

This is an example of why helmets have ear holes and Umpires have "little" voices. To avoid having to deal with that second situation, a well placed whisper into that players ear hole suggesting, "I'm not sure if you got away with a cheap shot on that one, but I'll give you the benefit of the doubt. If anything even close to that happens again, you'll watch the rest of this game from the team bus." can work wonders, as long as he believes you mean what you say.

I had a similar situation in a JV game. The only difference was the tackle was in fact holding the end, who completely stopped threw his arms up and said he was being held. Unfortunately by stopping and giving up the tackle was no longer restraining his opponent. I only had to tell the coach once.