To be honest my ducklings run free range with their mother since they were about 1 week old. Which means they eat the same food as everyone else (muscovy drake and ducks, geese and chickens), I. E. Layers pellets, a bit of wheat and oats and kitchen left overs. They also have access to a compost pile and a brook.
I border a council allotment site and my ducklings were going there every day to look for slugs with their mum untill the allotment people started complaining and I had to block the hole in the fence lol
Now they learnt to go in the brook and swim away for hours, looking for insects and other little creatures.
That's why I love muscovies!