I just want to say to the OP that my prayers are with you, and I want nothing more than for you to be able to enjoy that next trip in the land of the living smiling as your children have a wonderful trip!
As for everyone else, I have joked many times about having my ashes spread on the ground beside the view from my favorite resort. I know it won't happen, not even sure how they would get my ashes through security at the airport....doesn't matter. I will be with them in spirit through every visit to my happy place if I'm not with them. Where my remains are makes no difference. An urn shaped like the castle would be awesome though, so I can be at home in my happy place forever!
And OP, I love the balloon release idea, and when you recover from your operation, you should release one balloon yourself from your favourite spot at WDW to give thanks for being able to be there! Happy, positive thoughts!