If all they want to do is the fast rides, then I would say that Disney in general wouldn't be a place to go. I love thrill rides myself, but I also really appreciate the other rides, because they're so well designed. It's more about the experience than the trill really. All the parks has a couple of thrill/fast rides, but not many. And I wouldn't say Epcot is any different than the other Disney parks. Personally, I love Soarin, Test Track (haven't tried the new one yet) and Mission Space.
How did it go in the other parks last time you visited? Did they enjoy any of the stuff that wasn't "fast paced"? Do they enjoy the shows and the fireworks? Because the Epcot fireworks is really amazing and is something you don't want to miss out on. There's also tours you can do, like the Aqua tour, where you get to swim in the tank with the sharks, turtles etc.
I'll admit that Epcot isn't my favorite park, but I simply can't go to Florida without at least spending a day there.