How can I block just Facebook for free?

I do not allow my kids (ages 11, 12 and just turned 13) on Facebook at home and they know they are not supposed to be on it anywhere (they are not responsible enough nor are they old enough) but they are on the rebellious upcoming teenage phase in life. And of course grandma lets them do as they please so she doesn't have to hear them whine. How can I block Facebook on her computer (she knows I am going to block it and doesn't mind as she doesn't use it)? I have tried the "internet options, content, content adviser, and so on" it doesn't work. I mean it blocks facebook but then it blocks other sites (even those I tell it can always be accessed). I have also tried the "start, notepad c:\WINDOWS\system32\drivers\etc\hosts, search, so on" that doesn't work either. Until I figure ut out they are not on the computers at all, but I don't want to keep it this way as they do use educational sites and games (such as jump-start and abcya.com). Are there any other ways to block FB without buying software to block just one site? She uses internet explorer on windows xp. Thank you for any tips!