Epcot is our favorite park. DS has loved it since the age of 3.5. In World Showcase, there are performers and things the kids like, and the kidcot activities are fun too. We still have all the masks that DS has made.
We take a nap as well in the afternoon, but the nice part about epcot is that is always open until at least 9pm. You can go back in and have a nice evening there, and eat at one of the many good restaurants. The night time show, reflections of earth, is a definate must see!