If I had to choose three parks with 6 year old's I would go with MK, Epcot and DHS. I would skip the Animal Kingdom because of the shorter hours that, depending on when you go, may include a 5:00 pm closing time.
That being said, your original post states you are going for 5 days with 3 full days to visit the parks. If you don't have plans for one of the mornings you could probably visit just about everything of interest once in the Animal Kingdom between park opening and lunch time, especially if it is an EMH day. To add that extra day to your tickets would cost somewhere in the neighborhood of $6 - $10 per person so maybe you could squeeze in a day at three parks and a half day at AK.