Use a router with good blocking capabilities to their computer(s), where You set up a total blocking of all http/sites... and then gradually allow sites they ask for access to.
That would be the only 99.9% safe way unless a parent is always present.
Other than that tip, I'm guessing ICRA or RTA can point You to the best software today has to offer.
http://www.icra.org
http://www.rtalabel.org
.. or maybe
http://www.asacp.org