Every single duck I've had that has gone broody outside, that I was unable to find died to predators. There is a reason why we call them "Sitting Ducks"
As long as the ducklings can be kept away from large bodies of water I don't see any problem with it, of course a small container of water that they can bathe in would be recommended especially once they get older, just make sure they can get out with ease. Duck feed or a raiser feed would be...