We just got back from our trip with DD5 and DS3. We stayed at Pop Century and the kids (and adults) absolutely loved it! We stayed at POR last year when the kids were 4 and 2 and while it was wonderful, it was so huge we had to use strollers to get the kids around the resort. We also liked that Pop had only one bus stop compared to the 5 at Port Orleans. Pop had a great kiddie pool with fountains as well as a pop jet fountain. The Pop food court seemed more kid friendly and had frozen coke on the drink bar.
My kids are crazy about Epcot. DS3 especially enjoyed The Living Seas, that jumping fountain outside the Imagination building, the Imagination "playground" and Living With the Land. He liked Soarin' ok (DD's favorite) and did fine with Universe of Energy and Spaceship Earth. We met TONS of characters at Epcot this trip, especially in World Showcase. Both kids loved making the Kidcot masks in WS.
I agree with the previous poster about MGM being the least kid friendly. We went there mostly to meet the characters. They liked Playhouse Disney and the Muppet 3D movie and absolutely loved the Honey, I shrunk playground, but didn't enjoy Little Mermaid or The Great Movie Ride. DH loved the new stunt show but the kids could have happily skipped it.