My DD just turned 7. At ages 4 and 6, Epcot was a 2 full day park for her. There is no way that we could do it in 1/2 a day, and if I told her that was all the time we were spending there, I can guarantee there would be tears.
To give you an idea - Our typical 1st day is 1. DH and DD stop to see the characters at the front of the park while I run for fast passes for Soarin; 2. go by the Character Spot to see Mickey and friends; 3. Ride Nemo, see Turtle Talk with Crush and look at the Aquariums; 4. Ride Living with the Land; 5. Usually time for Soarin FP about this point; 6. Sometimes, but not always have time to ride Journey into Imagination or visit Duffy before lunch; 7. Lunch ADR at around 12:15 or 12:30 for either Biergarten or Akershus (DD's favorite WDW restaurants are 1. CRT 2. Biergarten and 3. Akershus in that order); --- by the time we are done with that, it can already be around 2:00 p.m. 8. Start at Mexico (or see Duffy first if didn't make it before lunch)- ride Donald Duck boat ride 9. Spend the rest of that day going from country to country visiting the princesses and other characters, watching the entertainers, doing Perry (Phineas and Ferb) missions (when she was 2, 3, and 4- it was the Kidcot stations instead of the missions getting stamps from each country and coloring); picking a pearl in Japan etc.
On Day 2, we go try to pick up the other side of Future World (Test Track), hit Spaceship Earth, Club Cool, eat at whichever of DD's 2 favorite restaurants we didn't do the first day, see whatever else we missed the 1st day in World Showcase (characters and entertainers we missed, pick up more Perry missions (DD loves those), and pick up anything else throughout the park we missed the first day (there is always a bunch) or re-do the absolute favorites.