Hi Kim!
We first took our son to Walt Disney World at age 2. It was such a magical experience to see the wonders of WDW through his young eyes.
Walt Disney World has four theme parks - Magic Kingdom, Animal Kingdom, Epcot, and Disney's Hollywood Studios. It sounds like you'll definitely be headed to the Magic Kingdom and that is a wise choice. You'll find more than one day of fun for a young child in the Magic Kingdom! If you can, allow two days for this park.
At Epcot, I would tour The Seas with Nemo and Friends pavilion where you'll ride a "clam-mobile" before entering the beautiful aquarium. Epcot is a great place to see characters. Visit World Showcase and look for Mary Poppins at the United Kingdom. Belle is often in France too! Grab a Times Guide when you enter the park for a listing of the characters available for meet and greets. Our son also enjoyed the
Kid Cot Fun Stops. InnoVentions, Journey into Imagination, and IllumiNations fireworks.
Hollywood Studios - Beauty and the Beast show, Honey I Shrunk the Kids playground, character greetings including Phineas and Ferb, MuppetVision 3D, Toy Story Midway Mania, Disney Jr. Live on Stage, Voyage of the Little Mermaid show, and the afternoon parade.
Animal Kingdom - Festival of the Lion King show, animal viewing trails, Character meeting spots, Kilimanjaro Safaris, The Boneyard Playground, Triceratop Spin, Finding Nemo: The Musical, the train to Rafiki's Planet Watch (includes a petting zoo), and the afternoon parade.
When our son was young, we spent the least amount of time in Epcot simply because the park is so large for walking and/or pushing a child in a stroller.
I wish your family a memorable vacation.