I had this same issue when I set up pc's for my step kids.
First, I made sure that the pc was up-to-date w/patches (I assume you are using a windows os)
windowsupdate.microsoft.com
Next was the anti-virus:
http://free.grisoft.com/
Then I installed a freeware firewall I've used on other pc's:
http://www.pcworld.com/downloads/fil...scription.html
The last install was a free program from microsoft called family safety. This forces the user to login to the PC to access the internet and based on their login credentials and the setting the parent has input, they get webaccess to the degree you've decided. It includes the ability to make white/black lists and has logging so you can see where you little snowflake has been surfing. Very good stuff:
https://fss.live.com/Default.aspx
If you have them connected to a router, you can do what I did next, which was to go into your access tables and set them up by mac address to be able to access the internet only during certain hours. Check your router documentation or you can ask me here and I'll try to help you if you want.
Good luck, I know how you're feeling