First of all, I would congratulate myself on having achieved such a lofty socio-economic status that my children have money to burn.
Then, I would discuss the physical, social, and moral downsides of smoking. This would *deep breath* include but not be limited to: the mechanisms of lung damage, the unpleasantness of living (and then dying of) emphysema, the link to osteoporosis (incidentally, my mother, a lifelong smoker, has pretty severe osteoporosis- diagnosed at 46), how bronchitis feels, how my mother couldn't afford Christmas presents for me as a child, the effect on fetuses, the link to prematurity, the effect on the public purse, and the issues revolving around giving money to companies that market cigarettes to Indonesian children and toddlers.
Oh, and it smells.