DuckPan: 18F is not really that cold to a duck, though that may seem unbelievable. As long as they are not showing signs of distress, they probably are fine. If they have never gone in their house it's tricky to get them to go now. Just put a lot of straw or loose wood shavings in the pen, block the wind as much as possible and feed them more. You might want to empty the pond at night anyway. That way, they would go over to the bedding. If the water is not frozen, and I'm guessing it's not or they wouldn't be in there (check to make sure they can get out and are not frozen in), they are probably actually warmer in the pond where it's over 32F. When I first got my muscovies, I often had to break the ice out of their water dishes in the mornings and they were always out running around in the snow. Until this severe drop occurred, they were fine. As I've read over and over again, ducks have their own down coats!
(My husband's coworker's geese are still out running around in −15F and will not go in their house, either. They're fine as far as she can tell.)