Just an educated guess based on the episode titles presented on Wikipedia, but perhaps it is "The Unicorn Trap"?
I'll have a look myself, but by the episode description you're giving, that title seems to be the most logical.
-edit- Not that one. I'm gonna go searching more in-depth.
I don't even think it's been an hour yet, and I've already found another episode with sneezing in it! Not the one we're looking for, but an accidental find. x'D
My sixth-sneeze-sense is a strong one.http://kisscartoon.me/Cartoon/Mia-and-Me/Episode-018-King-For-A-Day?id=28044 -- Not only are the king and queen sick and sneezy in the beginning, but in the culmination of the plot, the characters are in a location called the Sparkle Pollen Fields. Most of the sneezes are coming from the fairies, but the unicorn is sneezing as well.
~9:35-~10:18, with one from the redhead a little before this. There's more human sneezing from about 11 minutes or so onwards for the interested, but the listed section of time is where all of the unicorn sneezes are.