Sounds and looks like a possible niacin deficiency to me, I would try a liquid niacin supplement, and possibly switching them over to a feed formulated for ducklings, chickens and ducks have different nutritional requirements, and also, u may want to change out their bedding and switch to hay/straw or pine shavings, hard surfaces aren't really good for their legs and feet and can often result in injury and not good for their development process,, durvet vitamin b complex, its a liquid niacin sypplement, it works really fast, pretty inexpensive and most feed stores carry it.