Hi.
Young ducklings need plenty of protein and niacin (a B vitamin) for strong legs, and general tendon and ligament development. It's a non fat soluble vitamin, the body cannot store!
So it's essential in ducklings diet, and associated growth and development !

Tip(s) !
I feed the following to ducklings, after they have used up their 'yolk' reserves.
1) Feed them 'scrabbled egg,' !
No salt or butter... Just egg only...
2) Pet Caged Bird Seed. I use a Budgie seed 'Feed' that contains tiny mixed seeds, grit, and added vitamins and minerals. Here in the UK, one brand is called "Trill". Google it, and see if you can get similar 'all in one feed.' Or make a blend up yourself, of said ingredients (!)
* I also get wild bird seed, and grind it up a bit in a coffee milk etc.
3) Give them super soft green things !
Nothingy stringy or in long pieces.
I grasp garden lawn grass, snip the tips with a pair of scissors.
** Anything firm or stringy will choke them ! So cut, blend, trim before feeding it to them !
Applies to fresh or frozen peas, dandelions, clover, stems of grass seed heads, from garden, or from a wild verge. (Nothing sprayed with weed killer though. Check the verge status first.)
Sources of Niacin ocurr naturally in : Peas, pea pods (mange tout / sugar peas.) In Spinach, Unsalted Peanuts,
Sunflower Seeds, and in Mealworms !
All the above are fine in small, regular amounts spread, throughout the day.
* By having the above in diet, over and above of using chick crumb, or milled down mixed grain and corn (chicken feed/ scratch) means you don't need to give Niacin Supplements (Meds) or Brewers Yeast !
Use these when Niacin deficiency is diagnosed by a vet, or a bird shows leg weakness, or cannot bear its own weight. (This will often need injections of Niacin, as deficiency is to bad to be cured by amended diet or supplement use.)
Start a duckling on a good balanced diet, and you will have no issues !

Don't !
Give lots of water logged food to ducklings, or they will get diaorrhea !
Keep finely cut up lettuce, cucumber,
use of 'strawberries, melon, other soft fruit' to a minimum !
In the UK, we cannot give lifestock any household 'food scraps' due to what was feed to certain farm animals, and associated abuse of using food waste Think Horse meat Lasagne ! YUCK...
* I can hardly afford strawberries, or melon for myself, why feed these to ducks ??? ...They are waterfowl, that eat : grass, greens, grains, seeds, bugs, slugs, and invertebrate.... mainly.
(And, only these fruit treats, as an absolute minimum.) Fruit is full of sugar ! .... Are you wanting a fattened, obese duck ? Who waddles for all the wrong reasons !?!
* Unless it's for your table...
Then you feed your flock the right and appropriate levels of feed, to enhance growth AND welfare !

** At ALL Times ....
** Ducks need WATER to swill and wash their food through their digestive system ! Over and above that of Grit !
As they gulp so much liquid with their 'right diet, don't give them foods (see above) that give them soft poop !
Keep the Strawberries and Ice cream for owner consumption only...

Please give any of the above alongside plenty of fresh water.
I highly recommend using lidded buckets (mayo, baker, food supply type) often given for free, if you ask.
And, cutting a 'head sized hole' in the lid, so the ducks can drink, and flush their nostrils (nares) after feeding ! BUT not turn this water into a Mud to
go Slush Ploppy ...
All fowl need added Grit, and Calcium, when its age appropriate (!) For growth and Egg development.
*** Did you know that Spinach (Iron)
inhibits calcium uptake? Don't give laying birds spinach, if you have "soft shelled eggs."
Or plenty of Peas, as these too inhibit calcium uptake. Due to the amount of phosphate peas contain ! Double soft shell whammy, feed error !!!

If you feed spinach and peas.
Please keep Spinach and Peas.... for Babies. Or those not laying that Egg !!!

That's "All Folks" !!!
From me and Daisy Duckling.
Quack... Look I did a Selfie