Hi Heather,
How wonderful to be visiting
Walt Disney World with your little one at a magical time of year, when the resort is decked out for the winter holidays!
I suggest that you take a look at the link I provided above. This webpage provides suggestions for planning your Walt Disney World trip with a preschooler. On it, you can get a feel for the different attractions and activities geared toward little ones in each theme park. You can gather more information by watching the
Walt Disney World planning video.
Going to
Magic Kingdom for at least a day with your little one is definitely a good plan. Magic Kingdom has so much for young children to experience! Especially Fantasyland is full of great attractions for little ones, from classics such as Dumbo the Flying Elephant to newer attractions such as Under the Sea - Journey of the Little Mermaid. Extending an evening to go to
Mickey's Very Merry Christmas Party will allow you to see the park in a whole new way, where it snows on Main Street, U.S.A. and you can meet characters decked out in their holiday finest.
There is easily enough to fill a couple of days at Magic Kingdom with your little one, but you may want to consider the other theme parks as well. If your little one enjoys Disney Junior, you might want to check out Disney's Hollywood Studios. If you have an animal lover, a trip to Disney's Animal Kingdom might be in order. Finally, if your little one would enjoy an afternoon wandering countries of the globe, head to Epcot.
No matter what you plan, I am sure you will have a wonderful vacation, Heather!