I'm no expert, but if your ducklings are showing outward signs of a Niacin deficiency, it could follow them for the rest of their lives if not corrected early. I have read it can shorten their lifespan if not treated. I'm sure this is referring to severe cases though, and yours sounds mild. Do you have a way of posting pictures? There are a lot of really smart, helpful people here - you definitely came to the right place. Nutri-drench isn't going to hurt, but it won't help this particular problem. Have you tried using brewers yeast? Not bakers - brewers. Usually found at health food stores. If the legs are bowed, they may need more help than nutrition. Again, pictures will help with everyone knowing where you need to go next.
I'm glad you are here!
I forgot to ask - what exactly are you feeding them? Please list their main diet, brand name and type, treats and supplements. Young ducks will show outward signs of malnutrition very young, so there is a chance you may need to change what you are feeding them.