I am not much impressed by the 'libertarian' arguments against a ban on smoking in a car with children. The only argument of value is whether and how it could be enforced.
People should be aware of the real potential damage to children's health particularly when they are trapped in a small space and the air is polluted with toxic chemicals. Secondhand smoke contains more than 7,000 chemicals. Hundreds are toxic and about 70 can cause cancer. Put that in your libertarian pipe and smoke it!