We had the same scenario last fall, and we did one full day at MK, one full day at Epcot, and one day split between MGM and AK. We did 1 character meal, and one other sit down dinner.
Must-sees will vary according to your kids' likes and dislikes. At MK, we did all of Fantasyland and Toontown, most of Adventureland, not too much of Frontierland or Liberty Square, and all of Tomorrowland except Space Mtn. We got there at opening, and stayed through the fireworks.
At Epcot, we did Living Seas, TT, Ellen's Energy Adventure, Imagination pavilion, HISTA, and all of the WS Kidcot stations. We got there at opening, but left at about 7 pm- we didn't stay for Illuminations because we were pooped! Lots of walking at Epcot!! For this park, I'd definitely
rent a stroller.
At MGM, we only did a few things- VotLM, GMR, and DH and I rode ToT and RnRC. Other to-dos would be Muppets 3D, Playhouse Disney, and possibly B&B.
We also only did a couple of things at AK- Kilimanjaro Safari and FOLK. Other must-sees include the Boneyard and Dinoland.
If you are willing to get up early, stay all day, and roll with the punches, you can do a lot. I'd suggest reading up on these boards, as well as guidebooks such as Passporter or the Unofficial Guide. That way, you'll be able to get a good idea of what you will want to do, and what you can skip.
Have a great time!!
