If its really children the K9 web protection,norton dns and Parental control is enough,and if you need keylogger for some other stuff then I have no idea 
http://www.relytec.com
http://www.parental-controls-software.net/micro-pc-keylogger-for-windows.htm
http://www.spy-tech-monitoring.com/
Nikos said:Thanks guys but none of the above is undetectable. AVs recoginzes all of them.