If you're talking about blocking IP NUMBERS, at the ISP level, then the XXX idea adds nothing new. You can do that already.
But if people think that having their kid's computer set up to block *.xxx is going to work, I'm betting against it.
Oh, it might prevent a few accidents, like if Whitehouse.com was forced to move to Whitehouse.xxx, but that's about it.
The best way to protect kids is to set up a firewall that ONLY allows access to specific parent-approved web sites.
This works much better than trying to narrow down millions of web sites to "just the good ones".