Make sure they have deep dry bedding to sleep in. I definitely pile in the straw in the duck house deeper in the winter.
Make sure they have a good wind block. At my house the prevailing winds come from a different direction in winter than summer, so just be conscious of where your wind comes from and that your birds have a place that is sheltered from it.
Make sure they have thawed water. This can be done in any number of ways, but basically, check on them a few times a day and bring fresh water if needed. I've had the heat element go out in a heated waterer before, or the birds tip over and spill the entire waterer somehow. I definitely check on them a bit more often in winter just to be sure.
Ducks do quite well in the cold. I live in a cold climate where it can freeze any night of the year and my ducks fare better than my chickens. I wish I'd started with ducks, I might not have even gotten chickens (not that I don't love the little buggers).
I sometimes dump the dirty water bucket down the toilet in the winter so I'm not creating a skating rink outside or clogging my drains inside.
Honestly they will be just fine. Don't worry too much