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Open DNS is a good way to go and you also get some very useful security tools too but don’t forget Windows Live Family Center >> http://explore.live.com/windows-live-family-safety-child-kid-protect-filter-sites-chat-faq << which you download free and then give different profiles to each user on a machine. That deals with content type and also has access timers which are superb. I use a combination of both to keep my sons usage in check and keep him away from content that is unsuitable.
There really is no need for Avast to duplicate these services.