In school-run territory (which is, I presume, where you were), I'd be ready to expect just that, I'm afraid. Such is life.
Once I did, I suppose, get the better of such an incident (we were both stationary and I just got a slight knock on the hand). It was at (light controlled) ped. crossing just in front of the station. I seized the door and spoke right across the child, addressing the mother in the driving seat.
"TELL YOUR CHILD TO GET BACK IN HER SEAT. CLOSE THE DOOR. DRIVE ON. FIND A PROPER PARKING SPACE. PARK PROPERLY AND SAFELY. THEN AND ONLY THEN ALLOW YOUR CHILD TO GET OUT".She complied with my request
. At least, she drove on, child still in car, and appeared to indeed be looking for a parking space further down the road...