You should go out and buy a guidebook. Although I know it has its detractors, I personally like the Unofficial Guide, but since you are going off-season (good choice for a 5 day trip), you may not need alot of the tips, but it will give you an idea what other people thought of the attractions. The official guide also gives a good idea of all the attractions, but it will tell you everything is wonderful.
I agree that you can do AK in less than a day, but I thought Tarzan was a bit lame (sorry).
At MK, Alien Encounter is great, but if your kids are sensitive, stay away! Also like Splash Mountain and most everything in Adventureland. Although I know its a classic, I found Country Bears to be boring. I'm not much of a roller coaster fan, so I offer no opinions on SM and the train ride. Since your youngest is 9, you can probably skip the Toon Town Fair, unless you have time (and your kids are into autographs).
But, speaking of autographs (and I know I am probably going to get totally trashed here), I don't get it. You spend a ton of $$$ to get into the parks, and then you stand in line to have some kid in a mouse suit scribble "Mickey Mouse" in a autograph book. I personally think it is a big time waster. If your kids really want to collect autographs, take them to a Character meal or two. At least this way you can eat while you wait for the characters to come to you. (This is strictly my opinion).
I love EPCOT, and I think your kids would like it too, especially FW, but don't get caught up in Innovations. Here is another place you can lose alot of time waiting to play SEGA. In the WS, they probably won't get into the movies too much (France, Canada and China), but Mexico and Norway have rides, although not very thrilling ones.
Finally, MGM. Like I said, I'm not a roller coaster fan, but I loved ToT! I also have not seen Fantasmic yet (we are going in September, too, and this is #1 on our list), but I hear the Fantasmic dinner package is great, because you don't have to wait in the long lines to get in for the show (but I don't know what the lines would be like in September).
Have fun!