Well, yeah. They are children. They still require a bit more education and supervision, ethically and legally.
If injured, an adult can give legal permission to treat to the doctors in the ER all by himself. A kid can’t.
I’m not recommending getting parents’ numbers for tattling, but for emergency contact. I’m 33, and every time I go camping, I fill out paperwork with emergency contact info in case I die of poison ivy or get run over by an RV in the campground.
I’m so honored. 
At this point, I might as well say what our thoughts on the matter were. (Though actually, this thread has given me a lot to think about! Thanks all.) We were inclined to say yes, but haven’t done so officially yet as we were waffling between saying yes to all or limiting it to just a couple of friends.
It did occur to me last night that this may/will probably turn into a two night deal (not that it would necessarily be a problem if it did). They were asking to stay the night before - but if they’re going to be exercising all day, are they really going to be up to driving six hours to get home that night? I’m not sure that I’m comfortable with that, even if I’m generally treating them like adults while they’re here. We’ll probably invite them to stay Saturday night too.
While you may be less comfortable letting them decide to drive that evening than you would be with a similar group of adults, I see no reason why you couldn’t/wouldn’t say to an adult group “Um, you know, spending the day exercising and then driving six hours home sounds like a really long day to me. You’d be welcome to stay Saturday night as well.”
Yeah, that’s a good point, and as long as Risha makes it clear that it’s just “for emergencies, because if something happened to you, I wouldn’t have the faintest idea who to calll, and because technically you are minors, and I could get in trouble if something did happen to you and I didn’t have a contact number for your folks”, then I got no problem with it. But what I would have a problem with would be the sort of officious surrogate-Mom third-degree with pencil and paper handy, the minute they walk in the door, “Now, I need to know who allll your parents are, and all their cell numbers…”
Oh, sure. That’s why I suggested it happen before the trip. Also, 'cause, you know, there is a chance that one of the other parents is clueless about the trip, or thinks they’re staying at the house of someone they actually know, and I wouldn’t want to get caught in the middle of that. I like to make sure everyone knows what’s going on BEFORE I have an angry parent on my doorstep at 2AM wanting to know why Snookums isn’t where she thought he was.
Yeah. I pulled that “There’s a sleepover at Jodi’s house tonight although I won’t be there, I’ll be partying at Nick’s…” thing a lot when I was a kid. My kids will have to be WAY more creative than that. 
With them being skateboarders, I’d be more worried about weed than drinking, um, if my understanding of that demographic is still correct.
If it was me I would insist that they stayed Saturday night because I would feel horrible if I said they had to go and they were all tuckered out from skating all day and then got into an accident becuse they were too tired to drive.
You might sidestep the “give me your phone numbers” issue by getting them from MiL.
I know you won’t be able to search their belongings, but it wouldn’t hurt to discover a reason to pop into the living room occasionally throughout the night.
And I was the good kid.
I’d be really worried about them driving back the same day too. They’ll be in the city having fun, their scheduled late-afternoon departure could very well become late evening, and six hours in the dark is a long drive if your tired.
My comment about a curfew was just that if they’re not from Metro NYC they may not stop to think that it will take them more than 25 minutes to get back to home base. But I missed that this was only a 1 or 2 day stay, so it probably doesn’t matter. My mistake!
You can’t smell alcohol wafting up from your Aunt’s basement … 