I'm trying to imagine how it would have felt to her. You said it was just a few seconds after the toy went into the road that she ran over it, so it's entirely possible that she either did not see it, and even more likely she did not have time to react. And had she reacted - what were her options? Brake or swerve. Either of those action could have resulted in an accident, either the car behind her rear-ending her and giving her whiplash, or her swerving into another car (or into the child whose car it was, who could have been running into the road after their toy). Not sure, but I believe the advice in this situation is that it's safer just to drive on.
You say she returned a few minutes later, and was furious. Well, yes. From her point of view, she'd had a near-miss from a car accident. Her adrenalin would be pumping, fight or flight. And she was faced with a 12 year old, not a 6 year old. Maybe she'd have surmised it was a loss of control at 6, but your average 12 year old is pretty adept with a controller. 12 year olds, on the other hand - well I can think of a few who've been quite destructive around that age when bored. So, she jumped to the conclusion that the 12 year old had done it deliberately. That isn't completely unreasonable.
Your 'common decency' comment is at fault, in my opinion. She'd probably been quite shaken by the incident.
To expect her to telepathically divine that he'd lost control and not just been a bored boy looking for distraction and not valuing the toys he had is unreasonable on your part. Sorry, but it is.
I think you're directing your anger against her because you'd rather not accept being angry at yourself for not supervising him, or against him for being careless with his toy.