Well, a law could be passed about smoking in the home. If the perceived problem is that it's a parental decision, the same could be said about smacking kids and that didn't stop legislation being passed. The problem would be enforcement, but it isn't insurmountable. It would just be a case of the relevant authorities, be they police or social services, gathering sufficient evidence.
Another parallel would be TV licensing. That takes place in the home, but it didn't stop legislation making it mandatory for those receiving TV broadcasts, so it isn't as simple s it not being a public place.
As for cars, well, the parallel with using handheld mobiles is obvious, so that could work for smoking too.
Whether such a law will be passed is one thing, but I have no doubt that it could be, both in relation to cars and homes. How heavily it's enforced would largely be a function of the level of resources allocated to it.
Whether it should be passed, either for cars or homes is another matter, and that, IMHO, comes down to whether the medical evidence supports the requirement .... and my suspicion is that it does, where kids are present at least.
I see no problem in principle in banning it either in cars or private homes where kids are exposed. If there are no kids, then it's grossly intrusive, If adults don;t like it, they can always leave. Kids of then don't have that option.
I also wouldn't be supposed to see this type of law on the statute books, but I don't see it happening yet. But the attitude of the public to what is or is not acceptable changes, and the result is often legislation. Seatbelts would be one example, smoking in public places is another and attitudes to drink driving yet another. In each case, attitudes changed and legislation came along with it. Some of it, like seatbelts, we now all pretty much take for granted, and my bet would be that the same will happen to smoking, publicly or privately. The process is already well underway. It may take some more years, but in my view, it's coming.