My kids are older than yours, but maybe some of the things we do might be useful for yours, especially the three year old.
Each child carries her own backpack. In it are some little toys (ones that I don't care if they get lost - think Happy Meal toys), a favorite stuffed toy or two (usually a security animal or one they sleep with), a storybook, a magazine, a 3-ring binder I have made up, a pencil bag full of colored pencils or markers, an MP3 player and headphones, and some zip baggies full of snacks.
The magazine is a kids mag like Disney Adventures or Highlights, and I make sure I have one they haven't seen yet... usually this means pirating the most current one out of the mailbox before they see it, and hiding it till vacation time.
Each child has their own MP3 player (I bought really cheap plug-n-play ones at KMart) that I have loaded full of their favorite music. My 4 year old has Barney, Raffi, Wiggles, etc and I also make sure and add any WDW music they might enjoy like theme songs from Haunted Mansion and Pirates, the Small World song, the themes from Imagination, the Listen to the Land song, etc.
I have also made each child a 3-ring binder full of pages I have printed out from the internet. You would be amazed the number of age-appropriate print-outs you can find. My 4 year old's binder has coloring pages (Disney ones, also zoo and undersea themed ones to match the Animal Kingdom theme and the Seas with Nemo), florida-themed pages (beach, manatees, citrus, etc), some really easy educational pages (writing letters and numbers, etc), easy mazes and dot-to-dot pages, and other things that I know will keep her busy. i also bought severl sheets of little stickers at JoAnn's and put them in a sheet protector page at the beginning of the binder. She can amuse herself for hours peeling tiny stickers off and sticking them on pages of the binder. If you would like some links to places where I found print-outs, PM me and I will email you some.
Hope this helps, and have a wonderful trip!