Seriously, you've gotta try this:
When kids are too old to ride around the store in the cart but too young to be elsewhere in your eternal summer days while you grocery shop, do this: Give each kid a $10 bill. They are free to spend up to $2 of it on candy, the rest is for snacks and/or breakfast items that they generally harass me about unmercifully and I never buy. They will be gone for about 7 minutes and return with their sugar-laden bags and, depending on the kid, anywhere from $5 to $.07 in change. Then, when they continue on your trip with you and MUST have something, you can reply, "Do you have any money left?" Well, then.
The only drawback is when one child runs into a former teacher and explains, "My mom paid me $10 to go away." I'm expecting that phone call from CPS any moment.