I would tell them not to start. I learned this weekend my Dad's father died of emphasema (I thought it was heart problems) and my mother currently has emphasema. My father lectures me alot as he's never been a ciggerette smoker but has seen the consequences from his father and his wife - we had a long talk about it over the holidays.
As for other folks and what they tell their kids - I have no idea - one cute kid walked up to me years ago at Dennys while I was buying a pack out of the ciggy machine, he walked up and said - "ewww!! you smoke!! you are going to die!!!" I thought what a brat but I guess it's what its parents told him. I thought it was a little rude of the little kid but if that's what he beileves than good for him. Sometimes I wish I never picked up the habit.
However, I still think it was rude of the kid.