My vote is for 2 days at Epcot too. My kids love Epcot , and Ive said it before in other posts that the more we go ( just got back from our 10th trip ) the more time we spend in Epcot. Its too bad you dont have the hoppers, because we love hopping over to Epcot at night and walking thru the counties! Even if its just for 2 or 3 hours.
But since you cant hop.. definately 2 days at Epcot. I think one full day at MGM should be enough. Or if you have a half day at the end of you trip.. go there if you can. At ages 4 & 5, they cant do RnRc.. but they can probably do ToT. Beauty & the Beast on stage is great, Playhouse disney, Voyage of the Little Mermaid, and Muppets 3-D are all great for little ones. My kids like Indiana Jones, but there is gun fire, explosions, fire, and its loud. You could do the backlot tour if you have a lot of time, but that takes a while to get thru. See the characters too! Check out Buzz and Woody back at ALs Toy Barn. Our favorite place for lunch is Sci Fi. The kids LOVE sitting in those cars to eat!! Dinner for us is always Pizza planet ( unless we are there more than one day, and then we do lunch here on the second day) Oh, let them play in the Honey I shunk the kids playground. Make sure they stick their hands in the big dog nose! He sniffs you!!

If you want to meet the incredibles, then you have to go through the Magic Of Disney Animation.. about 15 minutes long, and then you can continue on to Mr and Mrs Incredible. I read they are doing soft openings for the new stunt show. Go see if you can check that out too.
You kids might not like Indy, or the back lot tour. They wont like Sounds Dangerously, and who wants to be a millionaire is boring for the kids when they are little. Star Tours may be a little scarey too. If you plan out the timing of what you want to do at MGM, you can get a lot done. Try to get a times guide at the resort before you get there. plan out what time the shows start and when to see them. I found that we end up walking around missing shows.. and its a time waster. Especially with one day.
Epcot.. its great to have NO plan there.. and just walk and check everything out. Kid cot stations at every country, making pressed pennies gets the kids excited to look for the new ones in each country too. There is a mini train set up at Germany and my kids love the hedge maze in UK. THe boat ride in Mexico is cute, and we love Maelstrom in Norway along with the viking ship there too. Epcot is Awesome.