Whatever happened to parental controls on software?

I'm intrigued by things that we need that vanish, particularly in software. It's an interesting form of market failure.

Ten years ago one could buy very good third party 'shells' for Windows 3.1 and Mac Classic designed to allow children to work in a controlled software environment.

Now there's nothing comparable for OS X and XP. I knew XP had nothing to offer, but I was a bit more optimistic about Tiger: Gordon's Notes: Tiger and parental control software,

Alas, Tiger isn't all that much better than Panther -- it looks like there are decent parental controls, but if you try to use them you soon learn that they are fundamentally unworkable. I suspect they never really tested them with parents and children. They're not useless, but they don't really protect the child's environment from the child -- and it's a pain to recreate that environment.

So OS X is better than XP, but not good enough. Annoying.