Ah yes filtering...
An ISP level block will never work:
* Too many false positives, far too many utterly safe sites blocked.
* Way too many false negatives (failure to block). I think the stat is that it would only block 20% of inappropriate content.
* What the hell is inappropriate content? I'd rather block graphic pictures of murder and shootings (which are OK in the US) and keep the more natural erotic imagery. Violence is generally considered very harmful (except by the US), erotica is generally considered not considered harmful (except by the US and control freak religious states). The important word here is "generally", what's OK for one child or one instance may not be the same for another.
* The age of a child dictates what is inappropriate. When a child is approaching puberty and beyond they really should be getting to see the world as it is - which is full of violence and sex. They wouldn't exist if it wasn't for sex and the modern "Western" world is far from angelic when it comes to violence now, let alone in the last 200 years or even within living memory of a lot of adults or the oil driven violence and conflicts. A child needs to learn about the world, not be so blinkered and protected from it that they start life as completely clueless, ignorant, useless adults.
* In a home there are people of different ages, an ISP block will hit them all. What's OK for the parents will not be OK for teenagers, what's OK for teenagers may not be OK for younger children.
* Parents should just learn to ****ing be parents. Parenting is not about dumping children in front of a television to be advertised at (often sexualised advertising, but largely crass basic mind control merchandising), neither is it about dumping them in front an Internet connection and hoping for the best. Parenting is about actively bringing up your children, having relationships with them (hahaha, teenagers) and being there for them, to lead by example and to bring them up to be well rounded, thinking individuals. Guess what? Teach children that parenting is something that you can do by being lazy and "trusting" it to the state, and see how quickly they'll have children of their own and how they deal with that.
* Children will find access to inappropriate (see above) content somehow - either through alternative venues, through lapses in filtering or through sites that find a way past the filters.
</rant>