Aaaaaaanyway, getting back on topic, there's nothing gray about a ban on smoking in your car with children, because
a)cigarette smoke is a known carcinogen with links to numerous health problems (including death)
b)opening a window does not actually prevent cigarette smoke from entering the entire vehicle, as any smoker or passenger already well knows.
c)children have extremely limited powers in preventing parents from smoking. Sure, they can ask the parent to stop smoking, but if the parent insists then the child is crap out of luck.