Pigeon foot is a common sign of a B3 deficiency, I'd definitely start them on the cattle B injectable as soon as you can, Dosage is 1ml a day, can be spread out over treats or administer orally with an eyedropper or small syringe. I'd avoid putting it in water since you don't know how much they are actually drinking and since they like to spill half of the water container as well.
Purina Flock raiser does have Niacin in it, but as a preventative measure for your other duckling's adding Brewers yeast or a poultry vitamin to there feed/water would be great.