Hi Tiffani:
This is a great question. You can ask this question to twenty different panelists and get twenty different answers. Luckily, I have no problem giving my opinion, which can hopefully give you some insight to make your decision.
I have an 8 year-old and a 7 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 I'm glad you will visit that park.
As for your question, 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, Soarin' (there is a 40 inch height requirement), 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 Agent P's World Showcase Adventure. We also tour the countries of World Showcase.
A number of people love Hollywood Studios, which features Toy Story Mania, Star Tours, and Muppet Vision 3-D. But my top two "full-day" Parks are Magic Kingdom and Epcot. Your children will likely enjoy all four theme Parks but my family tends to spend a little less time at Hollywood Studios than at Epcot. Of course when you visit Magic Kingdom and feel you really need to see more of it, you can always decide last-minute to visit the park a second time.
Enjoy your vacation.