I think most times the "surprise" is more for the parent's benefit than the kids. And often, when surprised, the kids don't give parent the type of reaction they were anticipating. I would never surprise a child who is going on their first trip to Disney World. For smaller kids (under 7), I think its important to watch youtube videos of the rides, parks, and hotels to introduce the whole idea of Disney to them. I would also watch all the major Disney movies with them in advance of the trip. Disney World is overwhelming. Give them some frame of reference to help them organize the experience. It will also give you a sense of what rides might frighten them and you can talk them through it. Same issues for older kids.
If the kids have been to Disney before, you might want to surprise them, but then you deprive them of all the anticipation. I get that parents don't want to be asked "When are we going to Disney" starting from a year out though. I have found that the sweet spot is about 8-10 weeks out. It gives them plenty of time to get excited and help with the planning, but isn't months and months away. The older the kids are, the earlier I tell them. I have taken various assortments of seven nieces and nephews to Disney for the past 20 years. We held special events to "plan our trip," including discussing restaurants, which park on which day, which parades and shows were not to be missed, etc. They loved this!
I think the worst idea is to "surprise" a school-aged child on the day of the trip. In their minds they have "plans" for the next week and missing those plans is anxiety-producing. Yes, going to Disney might be more fun than Sally's birthday party, but they were really wanting to go to Sally's party and now they will miss it. Or, they know that the social studies project is due next Wednesday, and they want to hear the teacher say in person that its alright to turn it in late. They also want to say "goodbye" to their friends (and pets) and make sure their friends know they are coming back (Save my seat in the cafeteria). I have also experienced that some kids are just better travelers than others. For some kids, not being at home is anxiety producing on its own without making it a surprise.
I am currently planning a trip for July with my 13 and 10 year old nieces. They have know about this trip since January. We have picked the resorts (split stay) and restaurants together. After some research, we have decided which day to attend which park. We have decided not to include a water park day after considerable research. After reading several reviews, we have picked Hoop de Doo over the Aloha dinner show or Mickey's backyard BBQ. They have done a lot of "on-line" research which has been super fun for them! But, I have also planned some surprises that they don't know about including the Pirates and Pals Fireworks cruise.