It sounds like either splay leg, which is caused by walking on slippery surfaces, or a vitamin deficiency. Make sure their bedding is something they can get traction on (wood shavings, corn cob, a towel, etc) and not something slick like linoleum or flat paper towels.
Also try giving them some poultry vitamins, which you can get at the feed store. Lastly, ducks are really susceptible to vitamin B deficiencies, especially Niacin. You might want to get some extra Niacin in their diet. There are a lot of ways to do this, but the easiest is to go to a pharmacy and pick up some Niacin from the vitamin section. Don't get anything that says "flush free" or "no flush" because that's the wrong kind for ducklings.
Chicken feed isn't really made with ducklings in mind, so they don't really get all the nutrients they need if that's the only food they have.