Ducks are not readily susceptible to anything, really. They're very hardy. The niacin won't hurt the mother if it's offered as brewer's yeast or from game bird feed mixed with their feed (but the protein will be high--which is probably okay as she's probably in need of it after brooding anyway). If you offer it as tablets from the pharmacy, in the water, she could overdose--I don't know for sure though. You could devise a creep feeder of sorts that only the ducklings can reach, and make sure the mama has pellets so the ducklings have to go to their own feeder... but the better bet is probably just to offer it as brewer's yeast or game bird feed if you can. Good luck!