As we lived in a small town with little grocery stores, I started leaving kids in the car for quick grocery trips if I could get one of the spots right in front of the store window once my oldest could unbuckle herself, open the car door, and wave her hand so I’d see her through the window and come out. That way she could let me know if they needed me. I think they were four, two, and an older infant at that point. A year or two later for the times I couldn’t get that parking spot. Maybe four years later for when we were visiting family in California, where you generally can’t see out the front of stores into the huge parking lots and people are much more into one another’s business.
On the idea of leaving kids alone at home, even when sleeping for short times—I had a family friend who’s five-year-old died when she threw up in her sleep and aspirated on it. It’s not just major disasters like house fires and car wrecks that can make this situation dangerous. Fairly normal occurrences can be disastrous for a little kid left alone. I don’t leave my kids at home alone until about eight or nine when they can be trusted to respond appropriately to various situations—don’t answer the door, call me on the phone if needed, or go across the street to the neighbor in the case of a major emergency. At that point I will leave them for short periods of time when they’re awake.