Thanks for the advice... Glad to know I am not in trouble!
I think you are right, surprising them is the way to go, and will be made easier since they already know we are going to Florida on vacation. So we can discuss and have a countdown to "our trip" not mentioning the Disney part of the program.
We are actually flying to Ft. Myers and staying at my mom's the first night (flight gets in at like 6:30 PM), then driving over eeeaaarly the next morning to get a more full first day. I am thinking about having a package from Mickey already at my mother's house. I could include a letter inviting them to come visit, along with autograph books and other small Disney gifts to be used at the park -- guest of honor name badges? epcot passports? I'll come up with some stuff to put in the box. My mom would totally go along with this: "A package came to my house for you, I guess someone knew you'd be visiting Florida this week..."

The only trouble I see with this plan is that they will never be able to sleep that night from the excitement!
I have seen lots of other ideas, either for surprising the kids about the trip itself or just little surprises once you are there. I love the thought of leaving little gifts on the beds at the hotel for the kids to find when coming back from the park -- I might leave a note and a little something from Pooh and Cinderella inviting DS and DD to our character meals. Also I read about people bringing glitter and sprinkling it either in the room or at the parks... "Tinkerbell was just here!" OR writing a teeny little note (printed in really small font on the computer) sprinkled with glitter and rolled up & tied with a little ribbon from Tink. If someone loved tink, that could be a great invite to come visit her. For a boy, maybe you could "find" a "Pirate map" with a big X marking the spot of Cinderella's castle or just a Mickey head -- then say, this map is leading us to the airport, let's go!
Also read about phone calls you can get from mickey or the princesses that say have a great trip! only $2.50! WAY COOL!!
Anyway, this is so much fun.
