Wow Shawna, what an amazing surprise! You are one cool customer for being able to keep this a secret. The little ones are going to be thrilled once they figure out what is in store for them.
There are so many ways to get the kids excited for their trip. I would look into getting
Disney Park Customized Maps as a great way to surprise them. If you order them now, you can have them for the bags when they open them up in the car. I usually couple the maps with some Disney stickers, pencils and even a good coloring book with Mickey Mouse and his friends is sure to get the kids thinking about a Disney adventure.
You can also enhance the surprise by downloading Disney park music and even some from the attractions. My kids always smile when they hear the opening to
The Haunted Mansion on the car stereo as we head over to the parks. How neat would it be to hear Peter Pan while you drive along.
Just one last tip. i would avoid adding confetti or any other really small items, as they tend to just fall to the floor of the car.
If you have any other questions feel free to ask me again. Have a great trip. Hush hush!