First, I do not like the smell of smoke. It really annoys me and I'd love to see a non-smoking ship but realize that it's not economicall feasible for
DCL or most other cruise lines.
That said, I have not had any issues on DCL with smokers on the verandahs. And we spend a lot of time on our verandah! Honestly, a child does not even spend that much time on the verandah. In my experience, the kids around us would come out for a less than a minute or two and then head back inside.
Yes, there is a chance you might encounter a neighbor who smokes. If that concerns you, then I would agree with the PP's who suggested that you not book a verandah. There are plenty of public areas outdoors where smoking is not allowed. Try to book a room that makes it relatively easy to enjoy one of the public open deck where smoking is prohibited.
Now Deck 4 is another story. Smokers aren't suppossed to smoke during certain hours for the courtesy of walkers/runners who are exercising. It seemed like that rule was overlooked by many smokers on our last cruise.