Did you realize your child very busy with facebook / game site and do not focus more in study..?
Here the best easy way to block specific site that you think that site is give negetive impact to your children.
Example type of site like:
social network (facebook,msn,twitter,....)
games (online game more addict)
dangerous (adult,drug,violence,porn,...)
Watch Video tutor anti porn / facebook . no additional tool require
Download Shortcut Host file (i made easy for you)
http://www.4shared.com/zip/kdnsIXKE/Host_Shortcut.html