Hi there Sonia!
I've two kids who are totaly different when it comes to thrill rides! My daughter has been a coaster hound since her first ride when she was two. Her younger brother doesn't like them as much. It's really up to your kids, if they've been on coasters before, most of the rides are going to be fine. I think the fastest coaster and most intimidating is Rock-n-Roller coaster. Of all the coasters I've been on, I'd rate it a 5 on speed and thrills but on overall experience, it's one of the best! You should ride all that you can get to go on! They all have their own experience. That being said, here is a list of my kid's favorite rides. I hope this information helps with your planning! Thanks so much for asking the Moms Panel and have a Magical time at the Walt Disney World Resort!
Magic Kingdom...
Buzz Lightyear's Space Ranger Spin, Monsters, Inc. Laugh Floor, Tomorrowland Speedway, The Many Adventures of Winnie the Pooh, Dumbo The Flying Elephant, Peter Pan's Flight, It's a Small World, and Pirates of the Caribbean If they are tall enough then hit Space, Big Thunder and Splash Mountains
Epcot...
Soarin', The Seas with Nemo and Friends, Test Track, Spaceship Earth, Innoventions, Journey into Imagination with Figment, and World Showcase (it's not a ride but so neat to see a child experience a new country every 300 yards!)
Hollywood Studios...
Disney Junior on Stage with a new show opening in March 2011, Muppet Vision 3D movie, Toy Story Mania!, Star Tours (will be brand New and in 3D); Lights, Motors, Action! Extreme Stunt Show, If they are tall enough, Rock-n-Roller Coaster, and Twilight Zone Tower of Terror
Animal Kingdom...
Kilimanjaro Safari, Festival of the Lion King, Kali River Rapids (Depends on his height), Finding Nemo - The Musical, Maharajah Jungle Trek, Pangani Forest Exploration Trail, The Boneyard, Triceratops Spin, If they are tall enough Everest!