Hi Cheryl!
Congrats on the family first trip! When I was going on my first trip, each day counting down to the trip felt like an ETERNITY! So hold tight, it'll get here soon :-)
The Number 1 Park for Pre-schoolers is Magic Kingdom. From Fantasy Land, to Toontown (closing sometime in 2011), to Tomorrowland and Frontier Land, there is much to do for the little one's. My recommendation is to go to the Disney World Attractions link (click here to access), then on the left menu, click Recommended Ages and place a check mark on Pre-Schoolers. It will now sort all attractions at all the parks based on pre-school aged children.
For a guaranteed meeting of Cinderella and other Princesses, I would consider a Dining option, such as Cinderella's Royal Table (Magic Kingdom) Akershus Royal Banquet (Epcot), or 1900 Park Fare Cinderella's Happily Ever After at Grand Floridian. You're pretty much ensured to see Cinderella at all 3, with other Princesses attending, especially at the Royal Table and Akershus. There are some Meet and Greet's, but the best place for princesses are the Dining areas.
Have a terrific first trip and savor all the memories! It'll go by fast...