Do you want more parental control over your child's ability to surf the net?  Try using one of the sites listed to monitor and protect your children from visiting unwanted sites.

Buddy Browser: http://www.buddybrowser.com/Free-Parental-Controls.cfm

http://www.foolproofsoftware.com/ 

ZAC Browser: http://www.zacbrowser.com/ 

My Kid’s Browser: http://www.mykidsbrowser.com/  

KidZui: http://www.kidzui.com/  

www.gomcgruff.com (free) 

This solution is not a lockdown but a reporting tool that would give the parents a report of what the kid does when is out on the net.   It takes a quick install on the computer in question then sends the parent a report of internet-based activities done on the computer(s) in question.

http://www.netnanny.com/products/home  for Mac and PC.

http://www.smartkidssoftware.com/wdedm1b.htm.  You can not only limit internet sites, but also access to the hard drive and to preferences.  Seems like that would be a way to avoid a lot of potential occasions of mischief.