Some great information from the Consumer Product Safety Commission:
Every year, thousands of children are treated in U.S. hospital emergency rooms for falls from shopping carts. Falls from shopping carts are among the leading causes of head injuries to young children and most often occur when children stand up in the child seat or the cart basket.
Injuries result when children climb or fall out of shopping carts because the restraint system is not being used, children unbuckle or wiggle out of the restraint, or the restraint is missing. Children can also fall from the shopping cart seat even when an infant seat, infant carrier or a car seat is placed in the cart seat.
To prevent falls from shopping carts:
Use seatbelts to restrain your child in the cart seat.
Retailers should ensure that all carts have seatbelts and that the seatbelts work as intended.
Stay with your child at all times.
Don't allow your child to ride in the cart basket.
Don't place a personal infant carrier or car seat in the cart seat or basket.
Don't allow your child to ride or climb on the sides or front of the cart.
Don't allow a child to push the cart with another child in it.