My kids are older now, but when they were little I would make a judgement call and leave them if it was cool, if I knew I was going to be fast, or if they had been screaming for 5 hours straight and had finally fallen asleep. Apart from at the petrol station where I leave them in the car as imho walking kids through a petrol station is unsafe. If it is too hot to leave the kids in the car at the petrol station then I would time it so I could buy petrol at a cooler time of day.