Hi Angie!
  I know from experience that traveling with grand-kids is one of the BEST ways to experience the Walt Disney World Resort. You can absolutely utilize the
 rider switch option just as you described. However, not every attraction provides the rider switch option. 
Here is an attraction list that allows rider switch to help with your planing:  
  Magic Kingdom Park 
  Big Thunder Mountain Railroad 
  Splash Mountain
  The Barnstormer
  Space Mountain 
  Stitch’s Great Escape 
  Tomorrowland Indy Speedway  
  Epcot 
  Mission: SPACE 
  Soarin' 
  Test Track  
  Disney’s Hollywood Studios 
  Rock ’n’ Roller Coaster Starring Aerosmith 
  Star Tours® - The Adventures Continue 
  Twilight Zone Tower of Terror  
  Disney’s Animal Kingdom Park 
  DINOSAUR  
  Expedition Everest - Legend of the Forbidden Mountain® 
  Kali River Rapids 
  Primeval Whirl  
  Have a great time with your grand kids!
  Anne