1. You can either get the kid's club wristbands in the terminal or onboard. If there are long queues in the terminal or if you want to check out the kid's clubs when you board anyway, you can also get the wristbands at the kid's clubs shortly after boarding. I'm not sure how the Fantasy will work, but I'm sure it will be similar to the other ships.
2. There will be a dining changes/reservations place noted on the Navigator for embarkation. You can go there to make your requests. On other ships you can go to Palo to make reservations there as well, so I'm only guessing they will let you do this on the Fantasy too with Palo/Remy. It will be in the Navigator you get upon boarding. Otherwise, as a CM for the location and times.
3. Go to the spa to make changes to your spa reservations. Thy will be open when you board, just for this purpose.
4. You can keep checking the website for cabana cancellations - one will show up if someone cancelled theirs. Otherwise, onboard you can add your name to the waiting list at the Port Adventures desk. I don't think they have a waiting list online or via phone. Someone can correct me if I'm wrong.
5. The word so far is that an e-mail will be sent when the bookings for this become available. However, like the Chill Spa, it will likely only be able to be booked whilst onboard. Nobody really knows yet... keep checking the DIS for updates.
6. From the Magic and Wonder's 7-day schedule, and if the Fantasy sticks to it, Formal night is usually the second night. Semi-formal is usually on the second or third sea day. I don't know if anyone here can confirm which. You could always call and ask - they usually know. As for special menus, you have one themed menu for each restaurant, pirate night, formal night, welcome aboard, and till we meet again. (That's if they keep the same 7-day menus as before.)
I'm celebrating day 74 by taking care of a sick husband and a cat with pink eye. Oh joy! Lots of sleep and soup for one and a vet visit and antibiotics for the other. And a little DIS for me to keep me smiling. Only 64 days until we leave!!