For everyone in this thread that says a toddler won't understand the lecture, consider this:
Both of my grandfathers smoked; my mother's father liked cigarettes, my father's father preferred cigars. When I was approx. 3yrs old, I would apparently tell them both that smoking was bad for them and that they should not smoke (I have no actual recollection of doing this, but it has been confirmed by several older members of my family besides my grandfathers). Both of them, without speaking to each other and unaware of the other's situation, took this situation of a toddler telling them it would kill them as a sign from God, and both of them coincidentally chose the day of my little sister's birth (March 16, 1984) to quit smoking for good. Both of them quit cold turkey on that day and never looked back. Years later they would be shocked at the coincidence.
The moral of this story: it's never too young to teach children about smoking