I wouldn't. I knew of a case where a child (older than 6) was left alone in the car while their parent went into a building for several minutes, and in that time frame, the child had strangulated in a seatbelt and suffered permanent, severe brain damage. Someone passing by saw the child, disentangled them, and began CPR - this means the door must have been unlocked, so if the child had been left in a locked car, they might have died.
I also witnessed the aftermath of a child knocking a car into gear at my elementary school. The car ran into a soccer goalpost and was stopped in a ditch right in front of a busy main road.
There are just too many things that could go wrong.