Oh, ew... hmmm... wait... you got the duck in with the chicks... hmmm.... well... thinking........
maybe create a contraption out of a milk jug that the duck can reach in to drink, but not the chick. (brainstorming outloud here!) I would think that since too much vitamin C can hurt a person, too much of anything is bad... so the chicks probrably shouldn't have too much niacin. I had a rooster get in with my ducks from his box once or twice and drink the water when they were brooding... but that wasn't everyday all day.
Maybe give the duck some niacin water and spend some one on one time with him while he drinks... Ducks will always drink. You can put a bit of clover or grass on the water to get them to go at it. maybe that once or twice a day would be better than nothing.
just thinking!