With little girls that age, I'd say you could skip Epcot for this trip (unless it's YOUR all-time favourite must-do park). My DD didn't start to appreciate Epcot until she was 9 and even then it's only a 1/2 day park for her now -- I've heard the same from other parents of girls.
Whatever you do, do MK last -- otherwise when you're in the other parks they'll be asking to go there instead.
Epcot has been my daughter's favorite park since she was 5. She's 8 now. She ranks them: EP, MK, DHS, then AK. She loves Soarin, the Seas (esp. Turtle Talk with Crush), Club Cool, Journey Into Imagination, the foutains in the Imagination Pavilion, the character spot in Epcot (great place to get pics and autographs from the biggies), Maelstrom & the giant troll in the Norway Store, Akershus, pastries in Norway and France, crepes in France, mimes, Miyuki the candy lady, World Showcase Players, the beautiful Mexican pavilion, just to name a few. Oh, and I forgot the Kidcot fun stops. I don't know why, but she continues to love coloring those masks when we go. She really looks forward to it.
Last trip, we did the Kim Possible adventures and they were SO much fun.
Every kid is different. If I were you, I would take a look at the attractions in the other parks and see what you think will appeal to your kids. And don't forget to think about what will appeal to you. I think your kids will have fun in whatever park you visit, and it's your vacation too.
As far as doing in MK last, I totally agree with that. MK is kind of ride-ride-ride. In the other parks there are more shows and different kinds of attractions. So once they've done ride-ride-ride, they may not want to slow down and experience the attractions in the other parks.
Just my 2 cents. Have a great trip. You're going to have SO much fun!