@johari
I think some warmish water with Epsom Salts is the way to go. You're going to want that healed up quickly so that bugs don't get into it. Maybe some Neosporin without pain relief, Altho personally, I would not use that unless I needed to. And that also means that you're going to need to keep the little ones separate from the adults until they are much larger, bcz that is very likely to happen again. Fill up your sink, add the ESalts, 1-2Tablespoons per gal. Swish it about to dissolve, put d'ling in, and my bet is that d'ling will take to that, well, like a duck to water

let the d'ling splash splash for 20 mins or so. In fact, you might want to bring along 2 ducklings. If its still cold where you are, make sure that your ducklings are fully dry before they go back to the brooder.
And fwiw, I see your pic shows that you keep your kitchen utensils near your sink. Not trying to tell you what to do, but I'd remove all that stuff from that area straight away *before* the duckling's feet touch the water. I also think that after the bath/ESalt Soak is over, a wipe down of all surfaces with a Clorox + water solution would be prudent.
HTH