Yes, but it would be much more easy to control a closed network, then the entire internet.
Also
http://www.fightthedotxxx.com/ have some good information about the XXX tld. including
http://www.fightthedotxxx.com/Objections-Kids.html which shows that one of the main arguments for ICM registry have been to "protect" the kids by creating a XXX tld.... This argument no longer stands on its own, since the EU parliament want to have a .kid TLD in a closed network, which everyone would agree is a much much better solution.