Wow I am sleepy.
The first thing I usually suggest with sneezing ducklings is that they may have something in their nares (nostrils). So it is good to give them a warm bath (this is where duckins' question about age matters, as very young ducklings need warmer - not hot - water based on their...
Welcome to the Duck Forum!
There is alot to learn with ducklings, then they grow up and there's alot to learn with ducks!
The best way to start - once you join the forum here - is to read through the Sticky on raising ducklings...