K.V.
I only have one child, but sometimes I need to fish for my keys or load up something....so I've taught my son to stand there and he has to keep one hand on the car (standing right next to me). It's more like a game, but I remind him we do it to keep him safe - parking lots can be dangerous places. Perhaps this would work for you, as well.
I also like the thought of allowing the older child to climb in the car while you're strapping the other child in. My son also likes to climb in on his own, so I can see this as another great possibility.