If you are planning on spending multiple days at MK, then definitely go there first and ride Dumbo. That's perfect for kids of that age. The double edged sword with going to MK first is that the rest of the parks don't have as many rides/attractions, so they may not be as cool to the kids after experiencing MK. But to some folks, MK is Disney and they want to go there first.
If you go to AK first, go on either the Killimanjaro Safari or TriceraTop Spin(It's the same thing as Dumbo, but with baby dinosaurs).
At Epcot, I would say the Seas with Nemo and Friends.
At DHS, by default I would say Toy Story Midway Mania.