I am always a "go ahead and try it and see" kind of person.
Toss them out and watch them. They will tell you if they are too cold (hiding in a huddle, and retreating to the warmest corner available).
However, if they have never been outside before, they might huddle a little at first, simply because of the wide open wonder of it all.
So.... I would get a good book, pick a day where you have an extra two hours, and sit and read your book and watch them.
Ducks in general are pretty hardy beasts. You could always set up a 'warm up spot' in their coop or shelter, where they can run if they get a bit chilled.
Another choice, is to have them outside all day, and bring them back inside at night.
Don't forget to make everything very predator proof, ducks are yummy.