I agree that the Magic Kingdom is the best of the four parks, especially for kids, but it can also be the most stimulating. There are lots of areas where many different musics/activities/people are doing many different things. My son has autism, and we found that the Magic Kingdom for him was difficult at times. I remember that while we were riding the carousel, they were also doing the sword and the stone bit, and there were plenty of people talking, plus the music from the ride. I could almost see his brain melting down trying to process all the sound.
Having said that, the Magic Kingdom has places where you can sit and relax, and I would absolutely take advantage of those places. We lingered a bit longer in restaurants, and took time out to sit on benches. We also went back to the resort for 2-4 hours every afternoon to give him a chance to have quiet time. He was a bit younger at the time, so hopefully it will affect your child a bit less.
If your kids like animals, I think the Animal Kingdom can be the most relaxed park. Some people might substitute the word "boring" in place of relaxed, but I found that my son responded very well to the slower pace. I also believe that sitting quietly and watching the animals was very relaxing to him. If sensory is an issue, I would avoid Dinosaur (I've heard it is VERY intense), and perhaps Expedition Everest, but most other things would be fine depending on how sensitive your child is.
In my mind all the parks can be either relaxing or intense, but I wouldn't want to come to WDW and not see the Magic Kingdom. We have a six day trip planned in January, and we are using three days for the MK. I expect the world showcase at Epcot might be relaxing, but it may not be very interesting to youngsters.
Good luck and have fun!
