I'll spell it out in more detail for you since you have never been to WDW. In Toontown in MK, there is a building which has 2 lines- 1 is for princesses (usually Cinderella, Aurora, and Belle(yellow dress)) the other is for fairies (usually Tink and 2 others). Those lines can get long, the fairies usually longer than Princesses. As someone said, your best bet for the shortest line is to get there before Toontown opens to be 1st in line for whichever line you want to get in. While Toontown is posted as opening at 10, it usually opens at more like 9:45. We made the mistake of getting there right at 10 one trip when we didn't know any better, and the fairies line was already an hour.
Oh, and there is another spot at MK where Tiana does meet and greets, if she is still doing them. It is over by Liberty Square.
In the afternoons at Epcot, you will find Princeses out in several of the different countries. The lines are usually not that bad. They do take breaks, so if you don't see them there or a line waiting to go ahead and get in , check again. You can usually find Snow White in Germany, Mulan in China, Jasmine in Morocco, Belle (blue dress) at one spot in France, and Aurora in another spot in France.
Have a great trip!!