You can split the dosage up throughout the day so you don't have to put so much B on a treat all at once. Ducklings usually never turn anything down that's green.
That's the right brand, Durvet. For a Two week old duckling, 1/2ml a day until symptoms improve will work. The two best ways to administer B3 is either over treats or given orally with a eyedropper/syringe, but since they aren't in critical condition, I think going the treat route would be...