I can't think of any folklore warning about evil children. I agree, there's lots of stuff where the demon takes human form but doesn't get it right (like the devil's inability to change his cloven hooves). I admit, these BEK stories scare the bejazus out of me, even now, and I came across them a couple of years ago. But they do have all the ingredients of Urban Legends; they're scary, they're detailed enough to be true; we can picture them happening. Even now, when I lock the front doors at night, I'm scared to look outside
just in case. And I know how ridiculous that sounds.
BEK do appear to be a new phenomena. The Muppets sound like a bad nightmare; I'm fairly certain I've had similar ones involving cute little black kittens wanting to come into the house, and when I've opened the front door to let them in, they turn into huge black entities with malice aforethought. Very vivid nightmares, where my mother has found me sitting up in bed, screaming the house down, actually miming the actions of trying to force a door closed with something pushing against me.
There also appears to be touches of Twilight Zone about it all, although I'm not totally familiar with all the stories so I'm not sure if they appear in them.
The other thought that comes to mind - I remember a few years ago now, the story of a woman who reported being chased by demonic bikers through the countryside. The police could find no evidence, but the woman was adamant. What they did find, however, was a leaking can of De-Icer in her car, the fumes from which (it was said) caused her to hallucinate something she was terrified of. Her son had recently bought a motorbike, and she was terrified of Hell's Angels type bikers.
Now pay attention - you see that whacking great conclusion over there? Well, I'm going to jump to it without the aid of a safety net
Both times the BEK's were saw, there were cars running their engines; once in a garage. Perhaps the fumes? Perhaps they had leaking cans of de-Icer? Perhaps they made the whole thing up? One thing is for sure - I would never have a 'Welcome' mat outside my front door, no matter how much I protest I don't believe in vampires (okay, not those kind of vampires, anyway). And I don't believe the guy/guys saw BEK. But that doesn't stop it scaring the hell out of me.