Hi Theresa,
I'm so glad to hear that you are planning at least one day at Magic Kingdom. In my opinion, it is the quintessential park at Walt Disney World and my absolute favorite. Since there is so much to do and see there, I think a second day there would be a lot of fun and that way if you weren't able to get to everything, you'd have the chance on the second day.
I highly recommend that your son do
Sorcerers of the Magic Kingdom, regardless if you do this park one or two days. This interactive activity around the park is included with the cost of admission and is a great way to keep his interest in areas of the Magic Kingdom that might appeal more to your daughter. It was my 10 year old son's favorite part of our entire vacation this past summer.
If you do choose to go to another park, I would probably recommend Disney's Hollywood Studios as it has several "thrill" rides that appeal to his age group such as Twilight Zone Tower of Terror and Rock 'n' Roller Coaster Starring Aerosmith. Also at this park is the Lights, Motors, Action! Extreme Stunt Show and Indiana Jones Epic Stunt Spectacular. Never fear, there would be attractions and shows that your daughter would enjoy, too. These could include Toy Story Mania and Beauty and the Beast - Live on Stage.
I hope I was able to help in your park planning strategy! Have a magical trip!
~Allison L