Ah, well.. ok. Anything written here in no way reflects on how I think you or your sons would behave (I'm assuming beautifully) or the vast majority of our guests have been. (They've been not only above reproach, but extremely helpful.)
So here goes. These are my expectations as a hostess with two boys, nearly 5 years old and 15 months.
First, for any gathering where I need to be the "hostess" (um, y'know, in quotes), I have a babysitter come for my younger son. The babysitter is his babysitter and unless it's been explicitly said out loud, is not any other child's babysitter. This means that the babysitter is not, in fact, responsible for a room full of children... just for my younger son. My first rule, therefore, is that if one brings one's kids, one is responsible for one's kids. (Can I just use "you" please? I don't mean you, Tigermoth, I mean the general "you"... it's just that one finds "one" so awkward.)
Ok, so the second rule. Anything my kids aren't allowed to do, yours aren't. This means playing the piano, even if you think it's a stupid rule. It's my house and well, we're just stupid that way and it gives me a headache, especially the way your daughter bangs on it. That's a really obvious one, of course, but you wouldn't believe the people that try and find the reason for the rule and then work a way around it.
My eldest son is allowed places by himself. This does not mean that if you feel your child should be watched in those places that I should have any part in the watching. While I don't want your child to get hurt, I have stuff to do and can't tag team keeping an eye on the boys. This paragraph needs a rule number three: don't make me watch your child for you.
And don't let them whine at me or ask me for things I can't give them or wouldn't give my sons. It's just annoying.
The best guests to our house have been the ones that have taken in my elder boy (fine on his own but finer with some friends) and explored his dirty fish tank, misplaced books and completely disheveled bedroom. They've played with the younger boy and gooed and cooed at him until he was full of content. They've helped me out, which is far more than I can hope for in a guest, but get it time after time.
So, see this post as the rant it was. It doesn't matter what gets spilled and it doesn't matter if stuff gets broken (I've had good guests who have broken stuff and bad guests that haven't). If you try and be good guests, you will be.