Am I crazy, or would it just be a lot easier, and avoid any kind of First Amendment threats, to just tell parents to watch their damn kids? You don't want little Suzy watching "Nip/Tuck"? Don't put a TV in her room. You don't want little Johnny looking up "panda suit fetish" online? Don't give him his own computer. You don't want Suzy and Johnny playing "Grand Theft Auto"? Don't buy it for them! And yes, they might try to do all these things elsewhere. So? Forbid it, and make it stick! It's called "parenting". If you can't manage that, then your kids have a lot more serious problems than seeing Janet Jackson's nipple.
If I were a legislator who was hellbent on crafting a law to stop kids from seeing porn on the internet, here's how I would approach the problem:
1. I must realize I will never stop non-US porn sites from abiding by US law or from being accessible to kids.
2. Write a law that US-based commercial porno suppliers must register their URL with a government database. That database would be available for free to all net nanny software providers.
3. Then it is on the parents to get the net nanny software.
4. It would be up to the net nanny software companies to compete on who has the best blocks to foreign porn sites.