On our trip last year (boys aged 6 and 7; dd aged 2), I'd say that we were a "mixed bag" and hopped around quite a bit. Our eldest ds will tell you hands down his favorite park is EPCOT. He loves both parts (FUture and Countries) and he would tell you in a heart beat that Test Track is his fave ride (and he's NO thrill seeker -- it took him 3 trips to WDW to build up the nerve and he nearly backed out on this last trip, but FINALLY got up the courage and LOVED it). I'd say second eldest would vote for a tie between EPCOT and MK (he still loves Buzz Lightyear, we must ride BLSS at least 5 times per trip! But he also loves the fireworks and funnel cakes at EPCOT). DD would tell you MK (she's young and it's all about the castle and those parades with princesses for her! Second runner-up would be MGM because of the shows.) DH would probably say MGM (a RNR and TOT lover, though he does still love Space Mtn and all the MK rides!), and I'd say EPCOT with MGM in a very close second (EPCOT has a serenity to it I just love, and MGM is just so varied and compact and fun!) AK just didn't hit the radar too well with our crew, but then, we usually go in August, and while August in Florida is pretty hot, in the AK "savannah", it just seemed even HOTTER! Pictures of us waiting for the parade looked like posters for heat exhaustion. THat said, however, I do know we will go back to AK, because it has my second eldest son's favorite all-time ride: Kali River Rapid.
If you stay at MK all day, one bit of advice is when the day gets hectic, pick a spot to relax and cool down. One of our faves -- we ride the railroad all around the park, sometimes 2 and 3 laps! Then, once we're rested, we hop off and begin again! The kids love to ridethe monorail too, and the boats -- take advantage of all the Disney transportation in its many forms, the kids just think it's another ride! We usually go now for 9-12 days at a stretch, but if we were trying to do a 4 day tour, we would probably go to MK for a day, EPCOT for a day, then split a day between MGM and AK (AK in the a.m., then over to MGM and fantasmic at night), then use the half day to go back to whatever you wanted one last taste of.... THat's my concept, anyway! AK closes early most days anyway, and MGM is a very compact, do-able park, I think. And the only other thing I suggest: have each family member read through the guides, websites, etc. and then select 3 MAIN MUST-DO'S (either a ride or show or event -- whatever within reason!) THen see to it that your plans include those for each person, and then tell everyone that anything else you cover during the trip is "gravy". It gets awfully easy to become so absorbed in seeing it all, you don't enjoy (or somehow miss!) the few key things that really appealed to you to begin with. Stop and smell the roses -- you're in WDW!
Good luck with whatever you decide!