"A ban would be the nanny state going too far"? People once described the law against drink-driving as an example of the nanny state going too far, telling people what they could or couldn't do in their own car. Yet most now accept it as sensible. Other than being about protecting children within the car rather than pedestrians and other drivers outside, this law would be exactly the same. And given the number of children it affects, the benefits for the nation's health could be equally if not more dramatic.
Adults choose to smoke, but children are trapped in the confines of a car with no choice over the fumes they inhale. While we legislate to protect adults both at work and at leisure, we fail to protect children in one of the situations in which they are exposed to unacceptably dense concentrations of second-hand smoke.