They make a small appearance in the original fairy tale, ferrying the children back their home. (They got to the witch's house without any need for such help. I fixed that.)
But then I sent her back to the place and the ducks said that they would of course show up again, and that meant I had to do something with them. . . . They weren't just going to ferry the characters around again, they wanted Meaning and Significance instead of local color and plot device, and anyway this was a fairy tale so all sorts of things could happen.
They were enchanted, of course. Simplest thing to do, and it kept them from taking over.