We pay at the pump for this reason. Some gas stations around here offer several cents off per gallon if you pay cash, it's not worth it to me to take in 3 kids, and I would never leave them in the car.
To DH and I, we'll go to a fast food place and spend 3x on a drink than saving and going to a gas station.
Honestly, if DH ever left our children in the car, I would never trust him to drive them around when I wasn't in the car. Sorry, but if his laziness/negligence ever harmed our children, that's one of the very few things I could never forgive him for.