Problem is if you ban smoking in a car for the sake of children or passangers then you would have to bann it in the homes as well as a car is an extention to your property and who would be able to inforce a ban in your home, I rtealy cant see police making random house calls to check your not smoking with children in the house.
The only way they could ban it along these lines is to do an all out ban with I doubt will ever happen as the government get way more money from tax on sales then they pay out for smoking related hospital treatment.
Plus would then open a whole can of worms about then you would have to ban cars to stop people getting asthma from polution and fast food to staop people getting obese and costing government money in treatment, banning alcohol and so on.
The only way around baning smoking in the car without people shouting about personal choice is to take the same view on it as driving while using a mobile, eating, reading, drinking (non alcoholic drinks) ie they are a distraction and cause you to take your hands of the wheel and your eyes and mind off driving.
Its the only realistic way about it.
Yes smoking in a car with children in it is discusting but thats more of a welfare and social services issue rather then a law one.
You cant use the exscuse to ban it in cars on the grounds that others may one day get into your car and there will be chemicals in the seats and such as then what about people who enter your homes.
Before you know it there will be lawyers writting up wavers stating " I agree upon entering your home and or car that I will not now or in the futer sue on grounds of possible chemical contamination from ciggerette pathegines" and no one can enter tell your covered