What a beautiful story, your wife is an extremely lucky lady and what an amazing couple to adopt your daughters.
Would it be possible to have a photographer present when you tell her your plans? When are you planning to break the news to her?
Her head is going to be swimming, the day is going to be a huge whirl - make sure you have a photographer and videographer following her around so she can relive such an amazing experience over and over again!
My plan (although it's only in my head at the moment!) is on arrival at the hotel on the 11th (the ceremony is the 18th) suggest a walk down to the beach (where it will take place) to watch wishes. My daughter knows what I'm doing, and I was going to have her say to my wife something like "wouldn't this be a lovely place to have a surprise wedding?" just for the reaction
