For the most part, cruising with youngsters can be alot of fun. It's always wonderful seeing everything through their eyes for the first time.
Of course I don't know too many people who want to spend 100% of their cruise with their child, as much fun as they are. So there will probably be a few times that you'd like to have them involved in the children's program, which can not only give you a little time to yourself to go to the spa, get some sun, get a facial, or just go to a show.
With that said, one of the best children's program we've ever seen was on the Allure of the Seas. Sometimes bigger is better and not only did they have a wonderful area for the kids, but there are many many amenities on the ship to keep everyone of all ages busy. Definitely worth considering. (The Oasis of the Seas is its sister ship and equally as good.)
Another really good program was on Disney. But while it's wonderful, it's also quite a bit more expensive.
Then again, at that age, everything is amazing, so no matter what you choose, she'll have a wonderful time.