We just got back and we did give our daughter (who will be 10 in 3 weeks) sign in/out priviledges and charge priviledges. It worked out fabulously and she has now proven her responsibility and maturity. I also think the taste of independence was a real highlight of the cruise for her.
We did agree to the following rules ahead of time and she stuck to them:
1)You must sign out with a friend.....never by yourself
2)You must have a destination in mind.....no signing out to wander around the ship aimlessly
3)You must have the lab page us with your destination..."Amanda and friend signed out to go to the movies", "Amanda has signed back into the club"
Anyway, it was the perfect way to handle it for our family, she loved it and we always knew where she was.
In regard to charging....she had full priviledges and we discussed what an awesome responsibility this was. The whole time she did not lose her card and all that she charged was 1 arcade card, 1 pkg of m&ms, and 1 sprite. Not bad, huh!