If William was down at the roadside and the parked cars were still around I’ve wondered whether he wandered over and got into one of his own accord….perhaps there was a pet in the car that attracted his attention.
It sounds like something out of a Hollywood script but after observing my own boys it is possible for a young child to open a car door, hop in, and close it, all done with little noise. We have a small 1990s Honda hatchback that was given to us by a relative for our older child to learn in and my boys, almost 3 and 5, love to play in it. They pretend to drive it, touch every control and go on pretend trips etc. It is one of their favourite things to do.
As the parked cars had the windows down, it is likely they were not locked imo.
Maybe William wasn’t ‘snatched’ off the street, he was already right there - in or at one of the cars. There was no planning. No-one was looking for him at the time and the driver was able to calmly drive off with William.