I would never, ever accept a family who smokes in the house/car causing their children to smell. This is child abuse IMO. When I was a child my Dad smoked around myself and sibling because he didn't know any better. Now with all the research we know second hand smoke can cause a host of health problems to anyone exposed.
I have a couple of parents who do smoke and the only way I know is that they have mentioned it. I have never smelled a cigarette smoke on them, so I have no issue with that, other than I like them and would prefer they didn't for themselves.

So I would just tell interviewing parents that I have a smoke free home and if they have the habit, they must not smoke before drop off and pick up as the smell does linger and I don't want my other clients to think I have a smoker in the house. No judgements, just asking for courtesy.