I also have a hard time telling you one over the other, but want to speak up for Epcot with young kids

When my boys were toddlers they LOVED Epcot, even more than they do now at 11 and 15. Granted some of the things they loved are gone but there is still so much there for kids to enjoy-some of it you just need to look for! In Future World you have Test Track and Soarin (depending on age and height), Spaceship Earth, Journey into Imagination, The Seas pavilion that has the Nemo ride, Turtle Talk with Crush which is so fun not to mention walking around the aquarium, Mission: Space for those a bit older, Living with the Land, a quick stop in Club Cool, and in the Innovations building Glidden right now has an exhibit called Colortopia and while not the most exciting thing in the world, my 11 year old spent about 30 minutes playing around there last Nov. Character meets in FW include Baymax, Joy and Sadness and Mickey, Minnie and Goofy in the Character Spot.
Then in WS you have Frozen Ever After and the Anna/Elsa meet, the boat ride in Mexico, other M&G's like Belle in her blue dress, Mulan, Aladdin & Jasmine, Snow White, Aurora in case you missed them anywhere (but unless you do Akershus you won't meet Belle in another park). Plus there are all the cool entertainers in the different countries my kids loved watching like the acrobats in China, Sergio in Italy and Serveur Amusante in France and the drummers in Japan; you can buy your kids a passport book and they can go around to a CM in each country and get it signed, not to mention the free KidCot stops in each country where they can color a character on a stick for a little break. Gives adults a chance to browse some shops

I know I just listed pretty much everything in the entire park, but I get sad when people think there isn't anything for kids in Epcot. It will be busy on a weekend during F&W and during EMH I don't believe Future World is open so you should check and see what you could do during those later hours. I agree with a pp who suggested making a list of the attractions in each park that your kids would like and either having them choose, or you choose based on the lists. Whatever you decide have a great trip!