If you buy your babies from a hatchery, you will need to dip their beaks in water to give them a drink first thing. A good way to to this is as you take them out of the box, and you count them to see if they are all there and healthy, simply dip their beak, and set them in the brooder you have set up waiting.
If you are going to feed a commercial feel, be sure you do not get a feed that has any medication in it. The medication is for coccidia a common parasite that chicks could get. Ducks are not able to have that medication as it can be deadly. The protein content may be too high for them also. 20% is all that is needed.
A good duck or turkey starter would be a good choice.
The duckling will eat a little, swish their beaks in water and eat a little more. Water is very important as they can choke on feed without water.
If you would like to feed your ducklings a homemade feed, here is a tried and true formula:
Feed 3x a day:
BREAKFAST: cooked oatmeal, covered with a little water.
eggs covered with a little water
DINNER: homemade whole wheat bread covered with a little water, or milk.
Tender young green leaves should be offered at each meal.
They also like chopped green onion,s and dandelion greens. Watercress is also a favorite.