I get that what I'm saying sounds absolutist and simplistic. I get that there's lots of influences on kids and peer pressure and whatnot.
But I'm also going by my own experience as a kid. You were a kid too once, yes?
My parents didn't yell or scream or threaten. They explained. Even as a child I understood why smoking was bad and what damage it did to lungs. By the time I was old enough to consider smoking or have any opportunity to do so, I understood lung cancer, and addiction, and that it's much easier to start something addictive than to stop. These things had been explained to me openly, without masking the ugliness of cancer or death or addiction.
So when I say TEACH your kids not to smoke, I mean that literally. Don't preach at them or threaten them or yell at them, because it doesn't work and kids tune it out. Don't wait until they're in middle school to show them the bad side of the world.