I think part of it depends on the timing you want. Day of? Advance? That might determine it right then and there.
Day of:
* wake them up day you are leaving and surprise them by:
* character call to tell them they are going to WDW;
* mickey pancakes, Disney music in the background and a surprise delivery from Mickey with some items for the trip and a written announcement
* treasure hunt (if you have to leave early, that's not going to work)
* tell them at the airport if you are flying and tell them before you are flying elsewhere
Advanced:
* by way of cake -- some folks have done reveals on a cake. Cake plus finding out you are going to disney is always good.
* treasure/scavenger hunt around the house -- this can be more leisurely and you can start off with broad clues about doing something fun, then going somewhere and finally with the Disney reveal
* use the Easter basket! You can do a combo treasure/scavenger hunt and Easter basket with eggs full of clues leading to a basket announcing the trip with some stuff for the trip for all of them (movies, flip flops, sunglasses, beach towels, swim suits, etc.)
* if you live near a
Disney store, you can go in and they will announce it in the store and on the screen there.
* get a neighbor in to ding and dash and leave a box on the front door from the main mouse himself and have it filled with balloons and a note that says they are going to WDW
I'm not sure there is a bad surprise for this kind of thing! Have fun!