I would say from some experience of having kids that age our first trip, definitely MK 2 days. It will take you that long to see and do all that you want to do there. Then kids have their favs and you go back and hit the high spots. MGM is not all big kid stuff. Take 1 day for all that MGM offers. AK is very spread out. See the shows there, do Kiliminjaro Safaris as soon as park opens, animals a lot more active then. Camp Mick-Min is fun for kids, also Boneyard little ones like as well. Lots of walking at AK. Epcot, try to catch what you think your family would be interested on Future world side. Then catch supper at a World showcase restaurant. This way you can be on world showcase side, take in the sights, sounds and smells while waiting for Illuminations. What a way to cap off a visit to Epcot. San Angel Inn and Le Cellier are 2 of our fav restaurants at Epcot.
Get a Birnbaums guide NOW if you dont have one, this way you can see exactly what WDW has to offer, in very good detail. That way, before you get there, youll know what you definitely want to see, but leave room for the little surprises, sometimes thats what makes a trip to WDW so special.
Most of all, Have Fun!