Thanks guys - always good to have other opinions. I must say, my first instinct was not to go but I think as I said in my OP that will just mean that we feel DSD gets the final say in where our holidays are and we're at her mercy.

So I think we'll go #3.

While I think it's pretty unlikely that she would have a party and trash the house, I guess the possibility is always there and I think for peace of mind we should take her key off her. Like I said, she's given us no reason for us not to trust her since she's been living with us (for seven months now) but before she came here she had been experimenting with booze, drugs etc. so I'd rather not put temptation in her way. I think we'll just explain to her that even allowing her to stay behind on her own is putting a lot of trust in her already. She won't be happy not to have her key but hey, at the end of the day she's only 16 and she can't call all the shots, right?

Besides, if she has the key, that leaves open the possibility that she'll tell the parent whose house she's meant to be sleeping in that she's changed her mind and she's going to sleep at another friend's house, leaving that parent to have to contact us and basically keep an eye on the situation. I think that puts too much onus on the other parent. Easier if we just take the key off her.

Of course, that situation's not foolproof, she could go out and get another key cut in the meantime but at least we would have done everything possible to keep her safe.