Hello there Kimberly!
You are going to be in luck! Of all of the characters, The Princesses are some of the most available.
First, don't miss the Rapunzel experience with so much more interaction and involves some story time and drawing (Rapunzel's favorite pastime)! When you arrive at the Magic Kingdom in the morning check the times guide of when the first meet and greet is. When we were there in early December, the first experience started at 10am. You will want to be one of the first in line for the interactive experience because, the first 10 -20 children get a little extra time with Rapunzel and Flynn. I won't spoil the surprise but have your camera ready!
Princess Jasmine and Aladdin can usually be found in the Magic Kingdom near the The Magic Carpets of Aladdin. I've also heard that she can sometimes be found in Akershus Royal Banquet Hall in Norway and Morocco in Epcot's World Showcase.
Princess Tiana and Prince Naveen can typically be found at their Meet and Greet location at the Liberty Square Bridge intermittently from 10am - 4.45pm.
Mickey Mouse, Minnie Mouse and the Disney princesses will be moving to Town Square in the location currently known as Town Square Exposition Hall in spring 2011. Until that location is ready, they will continue to greet guests throughout Magic Kingdom.
As always, make sure to check the Times Guide at Magic Kingdom for character appearance locations, parade, and show times. If you cannot find the character you are searching for, go by Guest Services and ask a cast member if they can call the "Character Hotline". Hopefully, they will let you know where they might be located. I hope you and your family have a Magical Time at the Walt Disney World Resort! Thanks so much for asking the Moms Panel!