Internet filtering for children (or other reasons) that would actually make sense

I was recently debating in another journal about new suggestions for internet filtering, ostensibly to prevent children seeing child-inappropriate sites. This is normally met with -- imho justified -- cries of doom. However, it does seem likely that there would be ways to approach it which would actually do some good -- if you, as a reasonably technical aware person, were proposing something, what would it be?

Suggestions:

* Not support political censorship
* If it requires a large investment of manpower (eg. great firewall) be upfront about where that comes from
* Should fulfil stated purpose of allowing concerned non-technical parents to protect their children from inappropriate content to at least some extent
* Should not be a massive expensive unworkable pointless joke
* Should be clear if it will work a country at a time (probably not) or be a small but incremental improvement over large classes of website.

Whatever the government is thinking about is almost certainly unworkable. But if there were something NOT ridiculous which could be suggested instead, that would actually be better than just "it doesn't work", or at least make clear to people who DO want a solution that it may be expensive.

It might even have positive side effects if (eg) pure spam domain names were caught in the crossfire.