I rather doubt a self driving car in the hands of a 7 year old could possibly be more dangerous than a manual car in the hands of a 7 year old. It's fairly trivial to keep kids from using an iPhone inappropriately. A simple password would easily stop it, or we can keep using physical keys.
7 year old kids can be kidnapped in non hacked vehicles. It's considerably easier to track down a stolen car from 2012 with OnStar than one from 1992 without.
Relavant HN article from today: http://www.bbc.co.uk/news/magazine-19660765. I don't see why DDOS would be the attack of choice for vehicles, but say it were to happen. DDOS attacks happen. They can be defended against (see http://blog.cloudflare.com/65gbps-ddos-no-problem).
For the foreseeable future cars will have manual overrides, and I can't see any attack or series of attacks, no matter how horrifying, resulting in 32,885 deaths over the course of a year.
Certainly there are many scenarios and details to consider, but the typical objections seem to be red herrings, strawmen, or just totally bogus.
Modern cars are 100% dependent on computers, yet this doesn't happen. Why would it begin happening in the future? Note that the Google car is designed to be autonomous, it has no dependencies on external systems - there is nothing to DDOS. Yes, you can blind/obstruct it's sensors in a variety of ways, but it's much easier to just aim a laserpointer to a driver's eyes today.
7 year old kids can be kidnapped in hacked vehicles.
Statewide gridlock due to DDOS.
Just looking at the flip side.