As I said earlier, make it the onus of ISPs to offer this. "would you like filtering with your service". Some already offer this. Also a sad fact of life there are going to be kids with parents that don't care. Mechanisms are in place for assisting such families, in the way of social services. Of course sadly they are not perfect, but that is not the point. As soon as you start putting censorship in the hands of governments or corporations you will start running into problems. It may also not be in the interest of governments to start heavily censoring the internet (many already do already, including the UK government), but to really restrict what their citizens do is a dangerous precedent for the health of any regime.
http://en.wikipedia.org/wiki/Cute_cat_theory_of_digital_activism
I have alot of experience with internet filtering solutions, they are not perfect, but they do what instruct them do, you have tobe pragmatic about what you set out to control from the outset. This takes a risk based approach. When I design such systems, I am looking at protecting an organisation, primarily from security threats. This means malware gets blocked first and foremost, ost of these systems work on a category based approach thereafter. Webmail is general blocked, as is file upload and peer sharing sites, because an organisation has to consider data leakage prevention. Porn is blocked most of the time, but not for pure security reasons, but because it's a risk to organisations because HR may need to get involved in cases involving employees viewing porn.
Of course because of this knowledge I have, (i have it listed on my cv that I am quite good at the internetz and everything) I may be somewhat conceited about the ease of setting up such filtering systems, but I really feel this a service ISPs should offer to those that want it only and it needs to be implemented in a careful and thought through way that considers proper usage scenarios.