We had a very bad experience with our then 3 year old on Spaceship Earth on our first full day at WDW. The ride broke down (or stopped?) when we were in total darkness. DS was so scared and I must admit it was a little unnerving. We sat there for about 3 minutes although it seemed like 30! Anyway he was so scared of anything "in the dark" the rest of the trip that it was a challenge. We had to drag him into Small World because he had been so spooked!
Your youngester might enjoy the aquarium part of the living seas, the playground area at Norway, the river ride in Mexico, Food Rocks, riding the boat in the lagoon, the mini train set by Germany etc....
I think there are many wonderful things for young kids at Epcot but it is definately a different kind of experience than MK. Also, the character bus is a lot of fun, bright and noisy when it arrives!