In my opinion this only causes a false sense of security. Parents will have opted out of p0rn and think they are save - which they are not. Not even remotely.
A ten year old can find instructions on the web how to bypass this filters in about two minutes. AFAIK they are using "Cleanfeed" in britain, which can simpy be bypassed by using a proxy or by using https instead of http.
DNS filters can simply be bypassed by using another DNS server.
And then we are already at the next step of censorship:
What? Information is available how to circumvent filters? Shouldn't that content then be blocked, too?
Not to mention the fact that filters are simply not accurate. And we are not talking 90% not accurate or 80% or 70%. Unless you want to block everything and then whitelist, only a fraction of websites will even be on any filter list.