My wife and I are big fans of Lenore Skenazy's Free Range Kids, but more than once I've wondered whether a bystander would call the cops on me if I were to leave the kids in our well-ventilated car for just a few minutes while I ducked inside the store or the library.
According to this site, 623 kids have died from heatstroke from being left in cars since 1998. From the bar graph in the middle of the page, there doesn't appear to be a clear upward or downward trend. The total from 1998-present is only about 3/4 of the number of kids who died from poisonings in 2010 alone. As tragic as these deaths are, it's a mistake to say "if it saves one life, it's worth it." I wasn't able to find data with a quick Google search, but my guess is that the risk of injury or death from a kid getting hit while walking in the parking lot--even while holding hands and looking both ways and all that--is as great or greater than the risk of a kid being injured while being left in the car.
"Kids dying in hot cars" looks to me like the 2014 version of the 2001 "Summer of the Shark" hysteria. It plays to some of our worst fears. It could happen to anyone. It ranks pretty low on the list of mortality risks, though.