My DD(7) was 6 on our last trip and Epcot is a 2 full day park for us. She loves it and has since she was 2. If your 6 year olds like Phineas and Ferb, they could probably spend almost a full day in World Showcase just doing the Perry missions. Those were a huge hit on our trip a couple of months ago. At age 3 and 4, she loved going from country to country to the Kidcot stations- the can get a stamp from each county on a coloring cut-out of Duffy or Perry on a stick, or you can get a passport for the stamps to go in. There are entertainment acts and lots of characters (many Princesses) throughout World Showcase. There are Nemo, the Aquariums, Turtle Talk with Crush, Living with the Land, Soarin (if the 4 year old is tall enough- DD was at that age), Test Track (again if tall enough), Spaceship Earth, Journey into Imagination, Club Cool (if you don't mind them getting sugared up tasting free sodas from different countries), the boat ride in Mexico, Malestrom, and more!