I don't believe that a child under ten, say, should *ever* be left alone in a car (or with other kids that age), even for long enough to "grab a coffee." I feel that ten is a perfectly sensible age for a kid to be home alone (I think that was what the Arlington PD told me once, when I called for a friend - old enough to call 911, not old enough to think you can take care of an emergency on your own). I am OK with an transit-experienced elementary school child being alone on the T. Or someone who has demonstrated safe street crossing going to a park or shop, depending on circumstances. We didn't generally ask, but I think, statistically, that a child is not safe in a house with a gun, even when the parent claims the weapon is safely stored.
I was very surprised to see that dads got more slack, because the impression I got while parenting a child (admittedly years ago) was that men are presumed to be totally incompetent with children, all evidence to the contrary. wouldn't that make them *more* likely to be turned in?