The grit helps grind up food. If you can't get grit, sand, crushed granite or even dirt will work. That is where they get it naturally. Ducks love mud! They swallow small rocks to break down food It is always possible there is something stuck in there and you can't tell. I would recommend give it something to help grind up food. Usually people feed ducklings crumble which breaks down in water. Since you don't have access to feeds, grit will need to be provided. If they get clogged up they don't eat as much and can stop eating entirely.
I would probably recommend giving a varied diet. Just like humans we need a variety of nutrients. You can always take a variety of foods like rice, peas, lettuce, fish, oats and such, mix it together and refrigerate or freeze the extra. Throw it in a blender even. Thaw it out as you need it. Right now make sure you include a high protein and naicin food on the side as well.
You usually want them to have free access to food as well. They can eat a lot for their little size.
Here is a full list of thing we give as treats, but may give you ideas of what else to add to your feed. Definitely don't do too much fruit, as too much sugar can be bad.
https://www.backyardchickens.com/threads/the-ultimate-list-of-duck-treats-and-supplements.242460/