I don't think you can use the fact that cigerettes distract smokers theory for banning it as some posters have claimed otherwise where would it end, all cars to be automatic, windscreen wipers and indicators operated by foot, loose hair should be fastened under hair nets, no music, dvd's for the passengers, no GPS, sun visors down at all times, no putting on or taking off of sunglasses,passengers (especially children) should not hold, eat, drink anything for fear of choking, spilling, no one should speak or move whilst the car is in motion, and let's face it kids would probably be banned as they are the biggest distraction of all.
Whilst smoking in cars is not the healthiest of pursuits, sitting in a car in summer, in traffic, windows open or closed is probably the most likely way for a child to get cancer because the level of cancer causing pollutants in the air is so high, this also goes for walking in major cities in said conditions, living near industrial complexes and recycling, electricity plants.
Then lets not forget the cancer causing properties we introduce to our children every day, baggies for food, plastic containers for food, tin cans and plastic bottles, juice containers, food wrap, tin foil, chemicals in the soil, gm contamination, radon under our houses, magnetic fields from our tv's computers and electrical products, microwaves and mobile phones, wi-fi and the em fields from endless miles of electrical cables around our houses , pollution from our cars and toxicity from our leather furniture or scotchguard type products, cleaning products and even our bbq's to name but a few.
Lets face it the level of oxygen in our atmosphere is nearly 50 % lower now than fifty years ago worrying about a few cigarettes should be the least of our children's health problems.