Never leave a child unattended in a car. The inside of a car can become dangerously hot very quickly. Do not allow children to sit inside a closed car without adult supervision.
Unattended children are targets for abduction.
Children may also accidentally release the car brakes and roll dangerously. Be sure the seat is approved for infants, and is a model that has not been recalled. For older children, over one year and more than 22 lbs., you can move your child to a forward-facing car seat, securely strapped into the back seat of your car. Make sure that you purchase an approved car seat for the correct weight and height of your child. At the age of five years or older, your child should travel in the back seat of your car with a seat belt and shoulder belt buckled at all times.