How computer-savvy are they? One way would be simply to add this to your HOSTS file (it's a simple text file which you can edit in Notepad or any other text editor, just remember to save it in that format after editing and not as a .doc file or anything other than plain text):
127.0.0.1 www.faceparty.com
If and when you want to unblock access, simply delete that line.
I think I'd be tempted to restrict the site using Content Advisor. Control Panel - Internet Options - click the Content tab, then Settings. It may ask for a password; if you haven't done one you can do it now. This is to prevent your little darlings going behind your back and undoing your hard work.