Except... the issue (for me at least) is not that someone could come in and do something, but that children that age SHOULD NOT BE LEFT ALONE! She's FOUR. Her siblings are TWO. Kids that age could get into all sorts of danger ON THEIR OWN.

My guess, and it's just a wild guess, is that some awful accident happened (electrocution, poison, a fall), the child died, and the parents panicked and were afraid that if they were open about it, they would be found negligent, possibly charged, and their other children taken away. So they covered it up. And then things snowballed, and snowballed, and....