Hi Heather,
This is a really tough question as I love all four Theme Parks for their unique attractions and story-telling abilities, but I understand your desire to spend two days at Magic Kingdom. I think that's a great plan and with the ages of your children, you'll easily fill two days especially while exploring New Fantasyland.
Because Disney's Animal Kingdom offers several shows and attractions for all ages, and for the
Disney Junior - Live on Stage! draw towards Disney's Hollywood Studios, my vote would be to eliminate Epcot from your 3-day itinerary. Epcot is LARGE and will likely tire little ones quicker than the other Parks.
My suggestion would be to check out the Disney site on my link provided, and sort the attractions by Park, and then by
Age Interest. The link will help you determine which Park will interest your family best.
Wishing you a magical vacation, Heather, and please do visit me again with any other planning questions.
Sherri