Well, now that I have suggested you consider it, I am going to change my recommendation. I apologize for not being more thorough before bringing up the idea.
Apparently although it is supposed to be antifungal, ** tea tree oil can be toxic under certain conditions ** so something less risky would be better.
Can you contact a vet or wildlife rehabilitator who can recommend a safer antifungal?
edited to add:
Having looked around a bit, I see a suggestion for treating athlete's foot with apple cider vinegar. I know from experience a little ACV in their water does the ducks no harm, and it is said to be good for them. So dabbing their feet with ACV a couple or few times a day should not hurt, even if they nibble on the foot after treatment.
Now, as to whether it will work in this case, I cannot say.
An epsom salt compress may also be worth trying, but that would be more of a pain in the neck, since you would need to keep them from ingesting the epsom salts. But I have read that people use epsom salts with their ducks, but carefully since (if I recall correctly) it gives them diarrhea.
