Hi Mary Margaret:
I have a five year-old and a four year-old and have been traveling to Walt Disney World with them since they were infants. While we have fun at all of the four theme parks, the one park we never can go wrong with is Magic Kingdom, so you've made a great choice there. And there is plenty for your family to enjoy for two days.
As for your second Park, I would choose Epcot for its number of attractions and variety of activities. At Epcot, my family always takes in Spaceship Earth, Living with the Land, Turtle Talk with Crush, the Seas with Nemo & Friends, the attractions inside the Innoventions pavilions (my boys are currently obsessed with The Great Piggy Bank Adventure), and the Kim Possible World Showcase Adventure. We also tour the countries of World Showcase.
For your third Park, it's a tough call between Hollywood Studios and Animal Kingdom. A number of people love Hollywood Studios, which features Toy Story Mania, Star Tours, and Muppet Vision 3-D. If your children are HUGE animal fans, you could consider Animal Kingdom as your third Park, which features a fun safari ride, Conservation Station, and a great playground - The Boneyard.
It's such a tough call. I would make the decision based on your childrens' like of animals. It may be there is slightly more for your young children at Animal Kingdom.
Enjoy your vacation.