If they are inside only and you give them produce, or any food besides duckling crumble, make sure to offer them chick grit as well. Most people put it in a bowl near their food.
My avian veterinarian also recommends switching to an appropriate diet formulated specifically for waterfowl when...
If they are making a mess of their water you can cut a hole in the lid of a plastic container. That lets them get their heads in, but not swim around. Keeping them dry and not chilled is also important.
What kinds of ducklings do you have?