Laura, How totally cool for you and your family!!
I think that would be THE BEST present ever!! Since I've been a
travel agent for 9 yrs now and Disney Specialist for 8, I think there's no place like Disney to take a family vacation. And doing the cruise will be awesome. These are the things I think worked best for my family and the shows/rides my family liked the best. We have 3 DDs ages 13, 8 and 5 in Sept.
At Magic Kingdom ...you MUST do Splash Mountain, Space Mountain and Thunder Mtn. Haunted Mansion is alot of fun and a traditional ride. PhilarMagic & The Little Mermaid are 2 fav shows for us as well as Cinderella's Celebration in front of her castle. This is a great opportunity to see many of the characters as well as get their autographs. If at all possible see the Spectro Magic parade and top it off w/the fireworks at night. Also the Wishes Parade is pretty good.
At MGM you MUST ride Tower of Terror and Rock N Roller Coaster and end the day w/Fantasmic. The Beauty & the Beast Show is pretty good too.
At Epcot we loved Test Track and Mission Space but it is a pretty serious ride so make sure those who ride can handle it. The World Showcase is interesting but my girls found it kinda boring.
Illumanations is a good show to close the park if you want to stay that long. My kids didn't like Epcot that much...not enough rides for them!!
Animal Kingdom was a family favorite. We loved the Safari ride, the animals, the gorillas and other animals in Asia area. We also LOVED The Bug's Life 3 D Show but my 4 yr old didn't like it. Said it was too loud and didn't like the spiders. The Rapid ride is awesome but be prepared to get WET! The Dinosaur ride is pretty fun too but loud and kinda spooky for little kids. You MUST see The Festival of the Lion King show. It was awesome!!
If we had only 3 days I would either skip Epcot or do morning at Epcot, evening at MGM. Full day at Magic Kingdom, full day at Animal Kingdom. Hope this helps and gives you some good ideas.