> Opening a moving car door is presumably quite difficult given the aerodynamic pressure on the body. So the car would have to be moving rather slow
Actually, no - at least not slow by my definition.
You’d have to drive over 130km/h, this is when you need some serious force to open the door more than maybe 10-20cm - but anything slower than that, it’s still pretty easy, certainly easy enough for a kid to open the door wide enough to fall out or get in serious trouble..
Source: 18 year old stupid me and buddies, doing stuff like opening driving cars doors, going over 100km/h