Depends on the family relationship and dynamics.
When me and dc stay with my aunt in the summer, we usually eat breakfast at hers (and I bring dc's main breakfast with me, but we also use some of her stuff like butter, fruit), I'll buy lunch out somewhere, and she'll cook dinner. I might buy dinner out one evening too, and I'll often pay her entry fee into whatever attraction we're visiting, although she doesn't always let me. No money changes hands, and we don't pay for her shopping. There's no way she'd accept money, but that's because she sees herself as the adult and me as the 'child', it's not a relationship of equals.
With another aunt and uncle, or my cousin we stay with sometimes, I would cook for myself and dc more often and we do stuff independently during the day, so there I would shop for myself and ask if I could get anything for them/the house. But if we eat together and they cooked for me, I wouldn't offer to pay. I would probably take them out for a meal at some point.
If I were staying with any of my siblings, I would definitely offer to pay for the weekly shop. Even when I've been staying with them to help them with childcare (so not a holiday for me!), I've bought the odd bits.
So I don't think it's a one size fits all shop/not shop binary choice, it depends entirely on the relationship.