Hi Jennifer:
This sounds like a wonderful vacation. I have two boys, ages 8 and 7. For my family on a six day vacation to Disney World, we would spend two days at Magic Kingdom, two days at Epcot, one day at Hollywood Studios, one day at Animal Kingdom. My family really enjoys Epcot, including Soarin, Test Track, and World Showcase, and we appreciate having two days to ride our favorite attractions more than once.
I think the best plan would be to budget two days and Magic Kingdom, two days at Epcot, one at Hollywood Studios, and one at Animal Kingdom. After your first day at Epcot, if you feel that you've "done" the park, you can change your plans last minute and use your second budgeted day at Magic Kingdom instead.
You can also add the
Park Hopper option to your park tickets that will give you even more flexibility by allowing you to enter all of the theme parks on the same day.
Enjoy your vacation.