My girls (7 & 3) love Epcot. The oldest has been to WDW about 10 times and has always enjoyed it at every age. It is educational, but at younger ages, that is still fun for them, so my kids have a ball learning about energy, communication, imagination, etc.
The Living Seas is a must - my girls could spend hours in there watching the fish.
Innoventions is fun, all kinds of interactive games for the kids.
Spaceship Earth is really neat - the history of communication.
Universe of Energy - cool info about where we get our energy including a portion where you ride though a prehistoric era with dinosaurs
Imagination - the kids will love this ride, silly look at the senses
Soarin - of course
Test Track - fun for the adults - my oldest rode it at 4 and loved it. There is a height restriction, so the little one won't be able to do this one.
Mission Space - I haven't gotten up the nerve to try this one so can't comment.
Then, the countries....
Where do I start - tons of food, cool stuff to look at, outside entertainment (like jugglers, singing, etc.), the characters are out and about here, and the best part for the kids..... the Mask Stations. Family Fun (the magazine) sponsors these Mask Stations. The kids get a mask and at each country they add another embelishment to it, the cast members in that country will stamp it (like a passport showing they were there), and if you ask will write your child's name in their native language. All for free!!!!
Can you tell we like Epcot?