We have been to WDW 3 times, each time DH and I as well as our children grown to love Epcot more and more. This year both kids rated it as their second fav park behind MGM!!!
First trip they were 3 and 6, spend 2/3 of one day there, it was ok. DH and I loved it but kids only wanted MK.
Second trip they were 6 and 9. We stayed at BWI so could bop in and out of Epcot for shorter periods. Got them going on a Kidcot Mask which they loved. We did most all countries but in a hit and run fashion. Only ate in one WS resturant but they loved character lunch at Garden Grill.
Last trip (this year) they were 8 and 11 and loved Epcot. We did lots more in FW and ate in Japan, Canada and UK with them. They did passports which we did not finish because they spent so much time exploring each country.
I would recommend the following that we enjoyed with our kids:
TT, MS (DS and I didn't ride but had fun in the interactive area), the whole Imagination Pavillion, SE, Living Seas, pin trading, water fountain and street shows like Jammitors. You might try Garden Grill for lunch.
In WS, I would recommend doing either passports or masks. My kids loved the shopping booth in Japan, they spend 1/2 hour pouring over Japanese Yu-gi-oh cards and Japanese candies! Try to eat in atleast one country's sit down restaurant. Watch the shows, they are great. We loved Living Statues, French Mime, Off-Kilter, Chinese Acrobats, Japanese Drummers!
Hope you have a great time.