Where is the parental or adult supervision???? i would have done stop the car, asked this kid where their parents were, marched up to the door and told parent their child was throwing rocks at passerby's.
I wouldn't!! Some parents would care, of course, but certainly not all. I once mentioned to a woman I knew that her kids had started a sand-throwing fight at the park, and a bunch of sand got in the eyes of a smaller child. She instantly puffed up and
informed me that her kids had
not started that fight, the "other kids" started it!!!! Never mind that I was there, watched it start, knew who started it, and took care of the kid that was 4 or 5 years younger than her bullies. Or that she knew I'd been there...and she wasn't.
Her kids were never wrong!!
Unfortunately, this is a very common attitude anymore.