It is exciting to surprise your kids, but, watch some
youtube videos of a Disney World surprise so you're prepared for every outcome. Especially if they've never been before, they don't know what amazing things are in store for them, so, it might be an awesome response, it might not.
One thing we were aware of also, was making up a "fake trip", since what if they prefer the fake trip rather than Disney? Then they are disappointed. So, we picked something no kid would ever prefer, a trip to the dentist.
For our second trip, we told our kids they had a dentist appointment and had to pick them up from school early. We brought their Disney hats and put them on while standing outside of school. Then, asked "are these hats you would wear to the dentist?" and they said "no" and I asked, "where would you wear this hat?", and they replied, "Disney". Then we asked: "Instead of going to the dentist, would you like to go to Disney?" Yeses all around! Lots of disbelief, lots of jumping up and down.
My son didn't believe us. Even when I showed him the trunk full of luggage; "It could be empty". Even when we arrived at the airport. Only when he lugged his own suitcase, obviously full, to the terminal did he believe us.
Boy were they disappointed 2 months later, when the dentist appointment turned out to really be the dentist.