May I propose a moratorium on grammar and typo corrections? People might stop posting.
Goofydadeo, how old are your kids? It's a little bit easier if they're 12 and under, you don't have to give them sign out privileges at the Lab, but for teens it's a hope and a prayer. I have heard of the post-it note system. Keep a pad and pen in the cabin (or on the cabin door) and tell your teens to leave a note when they change locations. It could work. Also, give them a curfew. If you're really serious about knowing where they are, you could try the walkie-talkie route. Problem is, that tends to annoy other people.
I've told mine that if she breaks any rules this time, we will seriously consider not cruising anymore. It's a lot of freedom for a teen, much more than they get at home, and they may not be able to handle it.