I am so sorry for all your family has been going through. My prayers are with all of you. I think it's a wonderful thing you are planning for your family and after so much sorrow and stress I'm sure this vacation will be much needed by all.
You know your wife best...I think the surprise it great, but to tell you the truth, as a woman with kids who has had some tough times as well, a one-day notice may not be the best way to surprise your wife. I'm sure it will take a long time for her to emotionally heal from all of this and maybe one more thing where she has NO control what-so-ever (though something good) may not get a totally postiive affect. I don't know, but I'm thinking I would want a day or so to at least pack what I want for myself and make sure I have all things I need in order. It's hard to explain, but I think a bit more notice for your wife may be better...plus she may also want to share in the surprise for your children.
This is just my thought based on nothing more than my own gut...Again, I think this is an amzaing and wonderful thing you are doing for your family and it is obvious how much you all love and care for eachother. However you plan to tell your family i wish you all the most amazing trip ever and the needed stength to get through this difficult time.
Oh, and one suggestion for trip is a 2hour fishing excursion...we've done it twice and are planning another one for this August...and we love t/ My son has a great time because you tend to catch alot of fish. We take it from YC (10am) and have fished in WS, in fron of DHS and by the EPCOT resorts. It's relaxing and fun. If you're interested there is usually a 20% off coupon in the Birbaum book.