You granddaughters will love it. My girls turned 6 years old on their first cruise and we all fell in love with it! The Radisson is very close to port and it is a good choice for the night before the cruise. We are staying over at Beach Club Villas at WDW (just because we are DVC members and we are using points).
The 4 night is a great choice for cruising because they have 2 chances to get the ship into
Castaway Cay (if it is bad on Tuesday, they try again on Wednesday....none of the other itenaries have this option). Assuming the ship docks at CC on Tuesday, you can relax and enjoy the island all day without worrying about getting back on board for the kids show (since they do it on the last day, and on the 4 night that would be Wednesday, the day at sea). Also, you have all day on Wednesday to pack up. On the 3 and 7 night cruises you have to get packed to leave right after returning to the ship.
We love the 4 night cruise. We have done 6 cruises (two 3 night, two 4 night and two 7 night) and we have decided the 4 night is the best for us. We can still have time for WDW afterwards and we get in all the things we want on the ship without feeling rushed.
I can't believe how close we are getting. Thinking of our trip will get me through our long cold winter!
DJ
P.S. One thing I did for my girls when they were younger that they LOVED. I went to Claires in the mall and bought them each a tiara? (crowns) and I gave them to them when they were dressing for dinner. They thought that was SO cool!