Hi! I know there will be DISers much more experienced than I who will respond, but I got here first!
Your kids are just a tiny bit older than mine. My youngest will be celebrating his 4th b-day while we are there. My first thought would be how tall is your youngest? If he is 40" then he is good to go height-wise. Then you just have to decide on how he is for the 'scare' factor.
If you go to the Disney website, you can make a personalized map showing all the places you want to see, and you can read about each one, and there are some videos/photos of all of the rides, too.
http://disneyworld.disney.go.com/wdw/parks/parkLanding?id=MKLandingPage
Happy Planning!
Oh, and don't forget to pick up a Birthday Button at guest services!