Do you think you'll go back to WDW in the future or is this a once in a lifetime (or at least a long while) trip? I agree that MK is the best park for those ages, but EP has always been our family's favorite, so I wouldn't skip it. It's nearly impossible to do all of EP in one day, so I'd focus on World Showcase (Nemo, Turtle Talk, Soarin, Test Track).
I'd suggest you look online at the attractions and shows for DHS and AK and choose which one to eliminate based on your family's interests. DHS has Toy Story, Star Tours, Disney Jr shows. AK has Kilimanjaro Safari, Nemo and Lion King shows, train to the Conservation Station (which is often ignored but was DS's favorite when he was younger).
With only three days, I wouldn't go to the expense of the park hopper. Hit the highlights of the three parks you choose, and plan to come back again!