Can't you do this shopping at weekends? Or first thing in the morning when they aren't tired and less likely to be grumpy?
Take stickers and reward them for a sticker for every shop they behave in.
Two minutes for petrol, on a forecourt where you can see them, which is covered by cctv isn't going to be a problem. Doing shopping is.
'In 2003, 42 children died of heat stroke because they had been left inside
a vehicle. In July and August 2003, 22 children tragically died after being
trapped in car trunks. Leaving children unattended in or around vehicles is a
serious problem. More than one thousand cases involving injury or death
have been documented so far. Those injuries and deaths were caused by
heat stroke, a vehicle being put in motion by a child, children being hit by
vehicles backing up, children choking while alone in a car, being kidnapped,
toxic fumes, activation of automatic power controls, or being trapped in car
trunks. Children should never be left unattended in or around vehicles.'
You may think your children are safe strapped into their seats but at some point they are going to learn to undo them, and it could happen at any time. I didn't think my 18mnth old could open windows until he opened it and jumped out from the first floor. Young children need supervision.
Please don't do it, even if nothing happens, you will get reported sooner or later, and people have been jailed for it.