Lemme play devil's advocate here for a minute, for both sides:
For a new law:
Does smoking create 2nd hand smoke?
Does 2nd hand cigarette smoke, when contained in a small, poorly ventilated area and breathed in for up to 1/2 hour at a time, cause health problems in children?
Is therefore, restraining another human being, even your own child, in a small, poorly ventilated space and forcing them to inhale a damaging and harmful substance for a prolonged time an act of direct, intentional harm againt that child?
Is a child not recognized as a person protected under law from undue harm, even from thier own parents?
Don't we already legislate against pretty much all acts of direct, intentional harm against a child (child abuse laws, child safety laws, child education laws, etc)?
If these statements are true, why would a law forbidding another direct, intentional act of harm against a child be out of the question?
Against a new law:
Given that the above is true, would it not also hold to, say, feeding your kids fatty foods? Can we then make that illegal? What about spanking?
Conclusion:
Its really a question of WHERE the line is in terms of what is harmful to a child, and how harmful it is. Without any evidence to support how much a child is harmed by second hand smoke in a car, it is hard to promote yet another law.
I would say someone should apply some scientific research and study to it, and if its compelling enough (such as, say, a %40 increase in sersious asthma, or childhood lung disease) then a law could in fact be necessary.