I would suggest 1 day at Epcot and adding a day at Animal Kingdom. While Epcot does have a lot to offer, you will probably find that most of the rides you kids are interested in have very short lines and you can complete them in a couple of hours. Yes, you can only fast pass test track or Soaring, but if you get there early, you can ride one and fast pass the other.
Each family is different, but my two boys (4 and 6 last year) loved doing the experiences in the parks. So for Epcot, they liked visiting the countries to get stamps on their Duffy and Perry(Kidcot). They also liked the Agent P missions, but at 4 and 6, this wasn't their favorite.
At 4 and 6, their favorite was the Wilderness Explorer badges at Animal Kingdom. (This year at ages 5 and 7, they still love that, but are a little more into Sorcerers of the Magic Kingdom). I would put Animal Kingdom in the middle of your week, as most of the rides and shows are longer and it always feels like a more relaxing day. Then you can mix in some of the WE badges.