I have cruised both. Some differences that quickly come to mind:
1. On
DCL you will rotate thru 3 dining rooms for dinner, not eat in same room each night as you do on Carnival, you will have the same servers.
2. On DCL there is no buffet available for dinner - dinner is available at the same place where the breakfast and lunch buffets are, but is service, not buffet at dinner. This is NOT an option on the first or last nights of your cruise.
3. No casino.
4. DCL shows - great shows, very different than Carnival.
5. DCL has movie theater (if a Disney Co. movie is released in theater, it is released on ship on same date).
Magic and Wonder are my favorite Disney ships (didn't like the Dream as much) - I wouldn't think your grandparents would have any trouble getting around.
6. DCL room service has hot items (depnding on ship - mac n' cheese, pizza, etc) - Carnival did not have hot items, mostly sandwiches.