I personally take an approach I consider similar to the V-Chip on TVs.
I put ICRA tags on ALL of my content:
http://www.icra.org/
By doing that, I've tagged even my deep pages as adult. So even if I have a warning page on a site and you bypass it with a deep link that has been spidered by the engines, the magic tag is still there.
At that point, it's up to the parents to show some responsibility and configure their kids computer to not pull up content they consider objectional.
The real underlying problem here is many parents take no responsibility for raising their own children these days. Sadly, they really would rather let the government deal with it.