I would suggest at least one rest day (or two half days, split up), less time at AK and HS, and more time at MK. When we went with our 4 and 6 year olds last year, we discovered the following. Of course, every child is different and your kids may like totally different things from mine, but in the interest of throwing as much information at you as possible,
here was our schedule and their reaction to it:
1 day at AK - Animal Kingdom was exhausting for them, even though they are exceptionally energetic and healthy kids. Walking it even once was hard. The best "rides" at AK, Dinosaur and Expedition Everest, were either un-rideable due to height requirements, or too rough for them to enjoy if they were tall enough to do them. We have a couple of passable zoos within driving distance of home, so they had little patience for trying to find animals on the trails and in the oasis. Kilimanjaro Safaris and some of the shows were excellent, but honestly, at the end of the day they still said their favorite things about Animal Kingdom were Triceratop Spin (a Dumbo-style spinner with dinosaurs) and the chocolate milkshakes they had after lunch. We spent less than a full day here, and it was more than enough for the kids. Everything they saw reminded them of something more exciting in Epcot or the Magic Kingdom, and they kept begging to go back to one of those parks.
1 day at HS - Hollywood Studios was all about Toy Story Midway Mania, Star Tours, the Muppets, and some live shows. (Even then, we had to be careful not to have them sitting in show after show -- they were getting so antsy from sitting around, and that's mostly what Hollywood Studios has to offer.) Again, some of the headliners (Rock 'n' Rollercoaster and Tower of Terror) were not suitable for them, so they were skipped. The Disney Jr. show was a bitg hit, as was the Indiana Jones stunt show. One day at Hollywood Studios allowed us to see everything that interested the kids (plus more that interested US), and we did TSMM three times (twice at rope drop, once later with fastpass). If we'd gone back a second time, it would have been just to let the kids do Jedi Training Academy.
1-2 days at Epcot - At Epcot, it was all about Future World. Except for the 3 or 4 ride-style attractions in World Showcase, there was very little for small children to do in World Showcase. (Granted, we did not stop at the Kidcot crafting stations or attempt Agent P's World Showcase adventure.) The kids could have ridden Mission:Space (the mild version), Soarin' and Spaceship Earth a thousand times: they were far and away the "big hits" of our Epcot days. We spent 1-and-a-half days at Epcot and it was perfect.
2-3 days at MK - The Magic Kingdom was the one place the kids kept asking to return, over and over. We spent a full 2 days there and although we did all of our favorite attractions multiple times, there were still things we never got to. It's also nice to spend multiple mornings there, as you can get to different headliners after rope drop, and to spend multiple evenings, so you can see the evening parade one night and the next night, take advantage of the low crowds on attractions during the parade, riding your favorites multiple times. We save the fireworks for the last night, so we can get the kids to bed at a decent hour (and be ready for rope drop) the other nights.
1 day to relax - We devoted one half-day in the middle of our vacation for touring the resort, swimming, and meandering around (capped off with a cruise down the Sassagoula River from Port Orleans Riverside to Downtown Disney, for a late dinner at Raglan Road). It was the perfect way to recharge our batteries, and the kids were just as entertained and delighted by an ordinary swimming pool as they had been by Splash Mountain.
Daily naps - Daily breaks were very necessary, too. After lunch every day, we headed back to the resort and spent a full 2 hours napping. My husband and I needed it as much as the kids did, and afterwards, we were all refreshed and ready to go. The only day we experienced any exhausted meltdowns was the one day we tried to skip the naps.
