They should be hitting their niacin requirements on that feed as long as they're getting enough. Are you feeding them free choice right now?
I've had a couple ducklings who've liked sitting more than I would care for too. Some will recommend supplementing their feed with brewer's or Nutrional yeast, but I'm honestly not a fan. Yeast has a pretty distinctive smell, powders up on the bottom and gets messy when the ducks get their food wet, and if you have enough ducks, it's a pretty fruitless endeavor trying to make sure everyone gets an even fill. A duck who eats off the top isn't going to get anywhere near the same amount of yeast compared to one who cleans off the bottom. Depending on the brand the stuff can get pretty pricey too.
Another thing that gets often recommended on here, which I find 100% more preferable, is Vitamin B Complex. Might be a bit harder to find depending on where you're located, but many feed/animal supply stores in the US carry it.
This is the brand a lot of us use. I picked up mine from Tractor Supply Co. It says injectable but you can just put 1 mL over their favorite treat and feed it to them orally. If your ducklings are less than a week old I think you'd only want to do .5 mL.
@Miss Lydia could probably confirm that. She's super helpful with these things. I'll give it to them once a day for a few days if I'm worried their legs are looking a little weak. Pretty happy with the results.