Hi, Charlotte! If these were my children, I'd definitely be going to EPCOT. My kids love the park itself, but if you are at least 40 inches tall, you can ride two of the most popular attractions - Soarin' and Test Track - on top of everything else they can do!
We could spend hours at Innoventions, with a special favorite being the Great Piggy Bank Adventure. They love the Seas with Nemo and friends, especially Turtle Talk with Crush, and they really enjoy wandering around to see the fish.
Another popular attraction with my girls is the Living with the Land boat ride. I thought they might be bored, but they really enjoy seeing the freshly grown fruits and vegetables to compare them to what we put on our own tables, and they always look for the Mickey Mouse shaped veggies. Another highlight for them is the small section of animals that features fish, alligators, shrimp, and eel.
You will also find that each country in the World Showcase has something going on, whether it's a ride on attraction like Maelstrom in Norway (recommended for ages 3 and older) or a movie like at the China pavilion. My girls also LOVE the shows - especially the comedy troupe in the UK.
I think any of the other theme parks would be awesome with kids this age - Animal Kingdom being another favorite of my girls - but I think EPCOT offers you the most to do. You can cap off your evening there with Illuminations, and know that you've had a day filled with lots of activities!
Have a great vacation!