Net Nanny costs between $39.99 / year to $89.99 / year, but I would prefer content-control software without a recurring fee.
I believe that something like parental control requires more than just an extension or filter. They are easy to get rid of and require some manual intervention. Precisely why, I've been using Qustodio from http://www.qustodio.com for a while now and am happy. Its free and allows me to block sites automatically in real time, track data, and also monitor the activities kids engage in on social media sites. The best part beeing that my kids will never be able to uninstall or disable its controls.
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