What nonsensical threats do you like to make to your children, or remember from your childhood? (Lighthearted please, no replies saying "never threaten something and not follow it through".) Here are some I remember:
Don't put your feet near the fire; you'll get them burnt off, like Pinocchio.
If you wear your trainers or wellies without socks, your toes might disappear.
If you put stones down that drain cover (under which rushing water was heard), a troll is going to sail along in his little boat, lift up the lid, and eat you up.
Witches (the child haters described by Roald Dahl) are real: the book clearly says it's not a fairy tale.