It's primitive and basic - by definition. There are only so many things that a child wants to do, and we've seen them do it a billion times. Add to this the FACT that only one in a million children can act and you reach a very obvious conclusion. For anyone who's ever made a film the logic of smoking in films is no mystery. It is hard to think of character defining actions.
Smoking acts as a short cut to a million emotional states.
Nervous, upset, reminiscent, angry, drunk, etc. In addition it is obviously imbedded in our collective brains that smoking is incredibly cool. There's not much to argue about - movie idols smoke. It's like trying to work out why James Dean is cooler than Tom Hanks - obvious and pointless. Anyway, the point is that smoking on film sets is a serious pastime.