Offer to help the coach reinforce his teaching of the rules by having a quick whistle whenever HIS team violates one of the rules he wants to emphasize. Then ask which ones hes particularly worried about.
I like the advantage idea with younger kids. If you really want to help fix bad habits that tend to linger, consider kids who play defense with their hands and hack forearms like they are choppin' wood. Also the highly effective foot fake with right, step with left and then dribble (and vice-versa) would be a terrific candidate. You might need to pack a light supper if you are going to call all of those though. Ah, the glory of youth coaching (and officiating, Im sure).
