a)cigarette smoke is a known carcinogen with links to numerous health problems (including death)
Not an opinion. The science settled this one a long time ago.
b)opening a window does not actually prevent cigarette smoke from entering the entire vehicle, as any smoker or passenger already well knows.
Not an opinion. I understand that it doesn't enter the rest of the vehicle from your perception, but no nonsmoking passenger in a car will support this. It's a smoker's fantasy and nothing more.
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.
Not an opinion. The parent makes the rules, feeds and clothes the child, provides the shelter, etc. That a child has limited decision making powers is not up for interpretation.