I feed my ducklings, well all ducks when there are ducklings, the same as for the older chicks. Grower chick food as it is not medicated. I also, feed finely chopped, Romaine, Kale, Turnip Greens, ect. As well as frozen or dried cooked peas mashed and watered down, their absolute favorite, thoughout life.
For the adults there is always oyster shell available, ducklings don't need it and don't eat it.
If you are giving grasses or greens remember to give them some grit, small grit for chicks is fine.
I just put a small dish out and let them get what they want. I use a lot of small kitten saucers and glass pie plates (heavy enough they can't tip it over as they grow) for food and water for the babies. Works great. Yes, some swim in the pie plate of water, but if their Moms let them, I figure they will be fine and they are.