Some folks get brewer's yeast (not baking yeast) 1 tablespoon per cup of feed. Or you can add Super B Complex to their water, using the amount of niacin = B3 as the guide. 100 to 150 mg niacin per gallon of drinking water.
For background, ducklings need 3 times the niacin chicks need, according to some sources. And from what I read on the forum, some ducklings seem to be fine, and some have leg problems or even seizures from low niacin and other nutritional deficiencies.
Your little one may simply have a sprain, but the B vitamins are a fairly simple precaution you can take. When my Runners occasionally have a sprain I put a tiny dab of T-Relief on their leg at night, rub it in really well. Takes several days for a sprain to get better.