I think if you look closely, the real kid is replaced by a dummy for the impact, and this is carefully disguised by the use of birds flying up in the foreground. With computer-generated imagery it's amazing what can be done these days. You have to suspend reality for the advert, because it claims that the car is doing 35mph, but it's a Nissan Sunny, and I've never seen one doing more than 30mph, even on a motorway. I don't believe that they are capable of 35mph so there is clearly some trickery going on there.