I wouldn't over analyze what you said, but I think I would try to focus on the positive of good technique. Instead of sayig you will die, say if you learn this you can huck really big waterfalls, and if you don't you can't (or whatever motivates them to kayak).
In 93' (when rolling instruction was primitive) my dad was trying to get me to keep my head down, and phrased it as slapping my head on my shoulder. That soon became, "slap your head down or I'll slap it for you." You should have seen the improvements over the next few roll sessions. I was 11. My dad is the shit.