I don't entirely agree Colin. I am certain there is some research which found that use of 'hands free' in cars is as distracting as using the phone in the normal way.
With regard to your analogies, I personally find trying to change the radio station or the tracks on the audio system quite distracting in itself, but I'm fortunate that there's usually someone next me being the DJ so I don't need to worry about it. I agree that children can be a very serious distraction, but I suspect most parents would say there are times when they have to have the kids in the car, so it's an unavoidable distraction. I doubt that most phone calls are so important that they simply have to be taken in the car. And to be honest, I strongly suspect that speaking to someone who is physically present is less distracting than speaking to someone on the other end of the phone - I think we're naturally inclined to need to concentrate more when the voice is at the end of the phone, and the person in the car is at least able to see if you suddenly come upon on a potential hazard and (if they've got any sense) shut up and let you focus on dealing with it.
Of course it wouldn't be practical to ban every potential distraction in cars and I'm not actually suggesting it would be right to univerally ban the use of hands-free calls in cars. No doubt there are people whose jobs etc mean that they need to make calls. However, my preference would be for people to think about whether they really need to take a call in the car and if not, perhaps just wait until they get to wherever they're going. I feel much the same way trying to fight my way through all the numpties on the platform at Waterloo walking obliviously along with their faces buried in their phones instead of looking where they're going. I can't help wondering if the email or text they're reading is sooooo important it can't wait until they've got off the platform and out of my way