Definitely, the Magic Kingdom! They will love Mickey's Toontown Fair, Dumbo, Winiie the Pooh's Hunny Pot Ride, Buzz Lightyear's Space Ranger Spin, Cinderella's Carousel, Country Bear Jamboree, Mickey's Philharmagic, Aladdin's Magic Carpets, Jungle Cruise, etc. Have a great time!
How could I forget-the great parades too-day and nighttime, if you can do it. Not to mention all the opportunities to meet their favorite characters. Try to do a character meal at the Crystal Palace with Pooh and Friends if you can. It's right in the park, so it will be convenient since you will only be there for one day.