Epcot was my kids favorite at those ages! If you have to skip something, I'd skip DHS. If it's between DHS and a 2nd day at MK, I'd chose the second day at MK. If it's between Epcot and a 2nd day at MK, I'd chose Epcot in a heart-beat.
Things they loved in Epcot:
Finding Nemo Ride
The rest of the Seas Pavilion (fish tanks, manatees, talking Crush)
Living with the Land boat ride (yes, really!)
Spaceship Earth
Imagination and the play place after imagination
The jumping waterfountains outside of Imagination
Everything about the Mexico pavilion and the boat ride there
Most of Innovations, especially the fire-putter-outer-game (forget the name)
The trains in Germany
Most of the Kidcot stations
The gardens in England
Character meet and greets in France and England
Things they loved in DHS:
The Honey I Shrunk the Kid playground. That's it.
Enjoyed Toy Story Mania, but nothing too over-the-top-favorite.
Walked out of Disney Junior show
Walked out of Muppets 3D