We were having the same problem with our DD12 when we first started talking about going on our cruise this coming August. We found another family that will be on the same cruise as us and their DD just turned 12 a few weeks ago.
We have been in contact with them via the Disboards and we now have their e-mail address. The two girls have been e-mailing back and forth for a few weeks now. The other Dcouple and their DD was on a cruise last year, and loved it.
The Dcouple DD told my daughter she will have a great time on our cruise. Another thing, the two DD's have a lot in common and have been hitting it off rather well via their e-mails. They intead to hang around together on the cruise.
Maybe you can find another family that will be taking the same cruise and see if they have a DD her age, maybe you can get in touch with them. I'm sure you've seen at the top of this forum about the unofficial cruise list. Check the cruise you are on, and she what other people are listed there, or start a new thread of your own, stating the dates of your cruise.
It worked for us, maybe it can work for you. Good Luck! We know what you are dealing with.
I do agree with ducklite, we told our DD she was going with us, whether she liked it or not.