Hi Jamie, thanks for your question! There will be plenty for her to enjoy in each park. Be sure to check out DisneyWorld.com's
Traveling With Little Ones section, which offers some great tips, including height requirements and special park maps geared toward younger visitors.
Even with the Fantasyland expansion going on, you'll still find quite a bit for her to enjoy in Fantasyland in the Magic Kingdom, including 'it's a small world', the carousel, Dumbo the Flying Elephant, Peter Pan's Flight and more. She also might enjoy Buzz Lightyear's Space Ranger Spin in Tomorrowland and the Jungle Cruise in Adventureland.
In Disney's Hollywood Studios, check out the Voyage of the Little Mermaid, Muppet*Vision 3-D, and Toy Story Midway Mania are all great choices. Disney Junior Live on Stage opens in early March so you might get to see that too. There is also the wonderful Beauty and the Beast stage show.
At Epcot, if she is 40" tall she can enjoy Soarin' at the Land pavilion. If not, you can still ride the Living With the Land boat ride. Head over to the Seas with Nemo and Friends, Journey into Imagination and be sure to explore the countries of World Showcase.
Over at Animal Kingdom, little ones love the Kilimanjaro Safari, Legend of the Lion King show and Finding Nemo the Musical. She also might enjoy Dinoland, USA and the kids play areas and games there.
March is an excellent time for weather, it's been in the 80's most of this week so that will probably continue in the coming weeks. Be sure to check the Times Guide when you get to each park to look for show, parade and fireworks times and decide which ones you'd most like to see. Wishes at the Magic Kingdom is a definite must-see!
I hope you have a wonderful vacation and that it's the first of many to Walt Disney World!