What a great Christmas present! You're kids will love it! I am doing something similar for Christmas although our trip is not until next Christmas.
Our entire family is going so I made Disney theme baskets for each person. I found beach towels that are the stick figures wearing mouse ears and had them monogrammed with each persons name on the mouse ears part. I ordered them from Disney Shopping toward the end of the season and got a great deal on them. I also put sunglasses, M&M tubes filled with pennies for pressed pennies, mini soaps, shampoos, new toothbrushes (Disney themed), tooth pastes, Disney Dollars, glow sticks, Disney crayons and coloring books from Dollar Tree, disposable camera, waist packs (if your children are old enough to wear them), hot hands(keep your hands warm on cold FL nights), and Disney themed t-shirts. If you haven't already, on the Creative Dis board, a very talented lady, Telynn, can design an airline ticket with your kids name on them that tells the date of when you are going, where you are staying...ect. They are sooo cute. It adds a nice touch to your gift. She can even do them Christmas themed with Mickey and Minnie holding candy canes with Santa hats on. Merry Christmas! I hope you and your family have a wonderful Christmas in Disney World! I'm jealous! I have a whole year left to go!