I'm not sure if this will help or not since I have no experience with 9 year olds overnight with 11-13 year olds!
However, I do have significant experience with overnight field trips as a middle school teacher, including 5 years taking 40 students across the country for a week. I have never had any of the problems that I think you may be worrying about- girl/boy in particular. My students are threatened extensively :-), including that if they are caught out of their hotel rooms past curfew, their parents will immediately be called to fly to DC and pick them up at their expense. We also duct tape the outside of the room doors so that we can see who has exited their rooms without permission and do night patrols. Those are 8th graders and we still have no trouble like that.
I think these experiences are invaluable life lessons and great social opportunities to make mistakes in a very controlled environment. I would want to know what the rules and expectations are, how much downtime the kids have, will they be constantly supervised, what are the consequences for violating the rules, what sort of problems have they had in the past etc.
By the way, my seven year old will go to sleep-away camp this summer for 2 nights. He's completely jazzed already!
