OK, get a guidebook, or look here in the planning section of the Dis. Find a list of all attractions in the different "lands" of Magic Kingdom. most of them correlate with a movie. make a list, watch those movies.
Here's what comes to mind for me, although i don't have a book in front of me:
Fantasyland: Dumbo, Peter Pan, Snow White, Winnie the Pooh, Cinderella
Tomorrowland: Buzz Lightyear, Monsters Inc, Lilo and Stitch
Mickey's Toontown Fair: If your kids aren't really familiar with Mickey and the gang, you need to rent/buy some cartoon collections that feature the old classic Disney shorts. Mickey Mouse Clubhouse would be OK, but doesn't really show the true personalities of the characters, IMHO.
Adventureland: none
Frontierland: none
In Epcot, your kids should see Finding Nemo, and (if they will watch it) Three Caballeros/Saludos Amigos. Your kiddos are a little young for the last, but they might enjoy the Donald Duck cartoon parts.
Hollywood Studios: Little Mermaid, Toy Story, Beauty and The Beast, Playhouse Disney shows. Also some exposure to the Muppets might be good. I know the season sets of the Muppet Show are available on DVD.
Animal Kingdom: Lion King, Bug's Life (if you plan on experiencing It's Tough to Be A Bug, which might be too scary for the little ones). Also, more Finding Nemo.
If mom and dad haven't seen Pirates of the Caribbean, I would recommend that too, but not for the little ones.

Also the Star Wars movies.