Here's my take. Raising kids is hard. And things will always go wrong. Every parent, I imagine, has experienced a scenario where they look away for a second and suddenly their kid is nowhere to be found. Or their mind just slips and they leave an exhibit while Johnny's still there. Or somehow little Caroline managed to worm her way into an out of bounds area (how on earth did she do it?). Or something catches the tyke's eye and he breaks away right into a street crossing.
These things happen to everyone. And most of the time, they don't amount to anything. The kid who wandered away in the supermarket is found a few aisles later in the ice cream section. An usher notices the kid by himself and lets him play cell phone games until mom and dad circle back. The girl in the off-limits area is found by a security guard and is ushered back. The kiddo in the street is quickly pulled back while no cars are coming.
But sometimes, through sheer bad luck, they happen in more serious scenarios. The forgotten kid is forgotten in the back of a car. The breakaway child gets into a dangerous enclosure. The lost kid is lost in a dense forest. In these cases, the consequences can be quite serious. But they're no more blameworthy than their more prosaic cousins. They're just more tragic.
My suspicion is that we seek blame in these scenarios as a psychological defense mechanism to assure ourselves that it couldn't happen to us or our family. If these things happen because the universe can be cruel and capricious, well, it can be cruel and capricious to us as much as to anyone else. And that's a scary thought. But if these things happen because parents are negligent, we can rest assured that they won't happen to us (because we're good, caring parents). A much more comforting thought. But probably not a very fair one.