Refresh My Memory #3

Here's another half-remembered horror tale from my youth:

A bad guy is holding a young boy hostage. (I can't remember whether he was a kidnapper, or a burglar who had broken into the house and found the boy alone.) I really can't recall anything of the set-up of the story, but I remember the ending vividly. The bad guy put a knife up to the boy's throat, as the clock struck twelve. Cut to an exterior shot of the house, with a scream coming from inside. Then we see the boy's parents arriving and reacting in horror...to the sight of their son, safe and sound in his lupine form, blood dripping from his mouth, in the middle of a bloody mess that they'll have to clean up. The boy apologetically says "Sorry, mommy, but you know how hungry I get at midnight." And as if that wasn't enough of a twist ending, there's an additional kicker: The parents have tiny little bat-wings sticking out of their backs, and the father says to the upset mother: "Now, dear, you know he can't help it that he's a werewolf and not a vampire like us!"

I'm pretty sure that I saw this story in one of Marvel's '70s horror-reprint books (like Chamber of Chills or Weird Wonder Tales), rather than a Warren magazine. Does this ring any bells for anyone out there?