Be sure to pick up chick grit also since your feeding other food besides their duck starter. Just put a small bowl close to their food and they will eat it as needed.
If nothing else works out put it in their water for now but if you can get some frozen little green peas thaw them in the microwave smash them a little put the liquid b complex over. Before putting the liquid b over let them get a taste for the peas and mealworms I haven’t met a duckling yet...
You can also use a dropper and put tiny drops right under the tongue one drop at a time so they don’t aspirate. It’s tedious but at least your getting it into them. Also any new food you introduce they will need chick grit. They have teeth to chew so they need grit to grind in their gizzard.