How much of the feed did they get? Is it that you received the ducklings from the babysitter, or was he or she just looking after the ducklings for a few days?
In any case, there are better feeds for ducklings than chick starter - as big a concern as medication is, deficient niacin is just as big for me.
Ducklings need three times the niacin chicks do. (There are reports that indicate to me that it may actually depend on the individual duck - some ducklings seem fine, others have serious neurological problems from low niacin.)
It's good you're so conscientious! I hope you get some good information and encouragement here at the duck forum!