For what it is worth, my kids really enjoy Epcot and our family found plenty to take up a full day at AK. I honestly think that some people miss some of the major attractions at AK (maybe they have confusing maps or because the names of various attractions are similar). By the time you do all of the "things" and see the parade it is closing time. I know the first time we went we totally missed the walk through thing with the tigers and huge bats and other cool animals. And then the safari, and petting zoo area and rapids and the shows, etc.
Even little kids seem to enjoy the Family Fun stations throughout the World Showcase, as well as getting their passports stamped and signed. I love the interaction with the CM's from other countries. They have several planned "meet and greet" sessions with the characters in Epcot now. Then there is the "educational" side which I personally tend to blow off, but the kids spent a while in the computer, photography, Innoventions area. The Land has lots of cute shows for small kids, Ellen's Universe of Energy is funny to most everyone, Test Track is great (not sure about your smallest, though, and not sure about Mission Space for anyone -- I can't handle spinning rides!), the rides at Mexico and Norway, Honey I Shrunk the Audience, Spaceship Earth, Illuminations, so much else.......okay, I'll skip to one of my favorites at Epcot -- the FOOD!
So, I would respectfully disagree about only spending a few hours in either Epcot or AK with younger children.