Wow. mixed gender sleep overs at 13? I am only 26 but still feel that is waaaaay too young. I guess it is up to what you feel you can compromise. I would go for a mixed gender party but the guys have to go home later and the girls can sleep the night. You can always chat with some of the other parents of the kids who would be going. I am sure a lot of them wouldn't like their daughters sleeping over when there are also guys. That way you can have a united front and at least know you aren't being too hard.