It took me an embarrassingly long time to figure out how to manage the grocery store with two infants. For my little guys, the best set up was to have one in the car sea tat the bottom of the cart and on in the structured baby carrier.
So, in our research, it seemed that lots of people were very satisfied with the Kindle Fire Kids Edition. It is cheap, runs a kid-friendly version of the operating system, and came with a thick, rubberized case.