No I wouldn't. What if someone took them? Predators are smart, they wouldn't just walk up to the kids and grab them. It's pretty obvious at our local playground which kids aren't supervised and that's because their parents are sitting at the cafe alongside it. So I reckon if someone really wanted too, they could find a way to lure kids out and it only takes a second.
I haven't read the whole thread so assuming someone has already mentioned this, but what about having a chat to them in the car before you go in to do the shopping and say to them if they are on their best behavior they can play on the playground for 15 minutes before shopping and they will get 1 treat each at the end. I find letting dd play at the playground for a little while before shopping really helps dd to behave and sit still. She is a lot younger than your two but can't hurt by wearing them out a little before you shop.