Have you ever done this before?
There is probably a perfect parent out there somewhere that thinks this is horrible. The rest of us Imperfect and Normal Families disagree. Because we have done this in some form ourselves.
The key is to keep the heavy things (like water bottles) on the bottom rack and the dangerous things (like fresh meat) tucked carefully under the baby's seat. Then that leaves plenty of room for bread and potato chips on top of your baby. As long as you don't block his or her airway, your child is likely to enjoy a comfortably nap, all cuddled up in a wonderful grocery cocoon.