I think it is important to make your account private, especially for younger kids. I found a former co-worker on FB and her preteen daugher was one of her friends. However, I was able to see her daughter's page. Since former co-worker is back as a part time contractor, I mentioned to her that I was surprised I could look at her daughter's wall. SHe didn't realize that the default wasn't 'private' and that you have to set it. She went home (we're not allowed access to the site from our work network) and change her daughter's and her own privacy settings.
If you are set to private, people can search for you, but they can't see your wall or friends without becoming your friend first. So, they would send you a friend request, which you would need to confirm.
I avoid having current co-workers as friends. If I want to complain about something that happened at work, I don't want what I said to get around the office. I do have a couple of co-workers as friends, but they are people I trust.