How do i limit time for individuals computers on Wireless network?

I have teenage children and want to limit the access to face book and other Social networking sites while they are doing homework. I am happy for them to use their computers for research but am finding that they are sitting on their computers for hours but little home work is being done.|||Try looking for different "Proxy Servers" these can block certain sites while leaving other sites completely accessible! the only way they could go on face book would be if you typed in a password that temporarily unblocked the site. just google "Proxy Server"|||parental control.Call were you got your internet service you will get the right answers and help from them. good luck.