If small children stick their leg or arm out from under the covers in their sleep, a window will open at midnight, and a fox will jump in and start licking their paws. The cub will become covered in fur, grow a tail, and become a fox cub. The fox will grab it by the scruff of the neck and carry it off to its secret den, where it will train it to become a cunning fox and forget its human life. Of course, it's possible to turn it back, but to do that, you'd have to find the den, find the fox cub who's already afraid of people, and lick its paw. No one has ever succeeded. So hide under the covers and sleep soundly, if you can.
And what happens if the child overcomes its instinctive fear of people and returns home? What's the chance that its parents will recognize it and not chase it away, consider it rabid, and send it to the zoo? What if they discover it was their missing son too late, when he's already grown up and his blue eyes have turned a wild yellow? Then they'll have to release him into the forest; licking his paw won't help.
However, a child can fool the fox by taking a stuffed fox into his bed and snuggling with it. The fox will carry it off, leaving the child behind.