I was the cashier, and every single purchase came to precisely 5 dollars, or 10 dollars. Noah kept coming to the cash with food, but no money. When I told him "These things cost money. You can't have them for free!" he would giggle, and then scan the room, where some money would be lying around, and return to the counter with his cash. L. kept coming to me and just giving me money. It was so cute - they were all getting bits and parts of the grocery shopping scenario.
After each purchase, I bagged their groceries for them. I even (though only momentarily) had them in line at the cash! They responded in good humour to my acting role as a cashier. I had to be firm with "rude customers" jumping the line, or taking others' purchases as their own at the counter. It was a real hoot!