AEvans I began letting my daughters use facebook and myspace when they were in jr high school with the stipulation that I had their screen names and passwords. They had to make their profiles private so that only family and friends had access and they weren't allowed to associate with anyone we didn't know. Anytime someone sent a friend request I would review it first to give them the approval to accept the request. If I felt it was questionable I would flag the request and pass my concerns to the site managers. I've only had to pass on one user name when myspace first came out and never had any issues after that. I kept this rules for all three of my daughters until they reached 16 yrs old and then let them use their own common sense. My youngest is now 17 yrs old and she is quite consequence of what she posts and what sites she visits.
Good luck with your children and hope it all goes well for you.