Hi Marcia,
Great question! Us Moms, just chatted about this very topic not to long ago, as one of the Moms surprised her two girls with a summer Disney trip. Her girls were so excited and she explained how she pulled it off and how great it was to see the surprise in their eyes and smiles on their faces.
Personally I have no willpower. I can not wait to tell my boys every time I plan another Disney trip. I have not even hung up the phone with Disney reservations, that I am not yelling, "Guess where we are going?". There are great points to both sides. There is certainly the joy you will get seeing their faces light up when the surprise comes. The look is pure joy and you will never forget their squeals of excitement. You will officially be the coolest mom ever. There is also the great plus on not hearing a million times a day, "When do we leave?" or " Is it time to go?" .
On the other side, I love letting the boys help me plan the trip. We have a great time looking at
www.disneyworld.com, pulling out our planning books and making our must do lists. The boys love creating countdown calendars with Disney stickers and pictures, and we often re-watch a number of our favorite Disney films to help pass the days. We create Mickey pancakes and cookies and decide what we want to do first. The boys fell it truly is their vacation too because they helped plan it and they look forward to it tremendously.
I would tell them now in some fun and exciting way like a balloon bouquet with a note from Mickey or a treasure hunt game that leads to a chest with Mickey treats. Then you can prolong the trip by planning together, making crafts, or autograph books, or just dreaming about how great it will be.
Wishing you all a magical trip!