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@Callender Girl says frozen peas is one way to add some niacin but you can't feed them frozen you have to thaw them first putting them under hot water and mashing them some would be good. But the easiest way for me has been to add Nutritional Yeast to their feed 1 Tab to each cup of feed for at least 12 weeks of age [this is when they are growing and need it to give them strong legs]
https://www.backyardchickens.com/threads/nutritional-yeast-vs-brewers-yeast.1393856/ if your ducklings are going to be a large breed they must get the extra niacin since their legs will be carrying around extra weight here is info for you
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
For housing you'll be alot happier if you make their coop where you can go in to clean or either put a roof on that you can remove to clean, Duck poop is wet and can build up ammonia so you want to be able to clean often and ventilation is another important factor when designing your coop there need to be good ventilation up around the roof area to let out moisture and some way to get fresh air inside becasue coops can get hot in summer months. Windows with hardware cloth covering them or make your own windows and use hardware cloth over every opening to keep out predators. Pine shavings is a good bedding becasue it absorbs their wet poop and is easy to remove then just add more as needed.
Good reading
https://www.backyardchickens.com/articles/categories/ducks.30/?page=3
I am sure you can find duck house info online but here is info on chicken coops here on BYC which many can be converted to duck housing
https://www.backyardchickens.com/articles/categories/chicken-coops.12/
Your ducklings are adorable feel free to share many pictures we love pictures. Also questions we are always willing to help.