Convert an XBox Live account to a child account?

So, I gave my autistic cousin an XBox Live card ...

I had made sure to go in and change his user profile so there was no correct information in it, but what I didn't know is that he's had a couple of incidents with telling people online where he lives (and in one case, someone came to 'visit').

Is there some way to convert his account so it's managed as a child account, so his parents can turn off voice chat, or set other restrictions?

The system wasn't set up for parental controls at all, and technically, he's older than the legal age of adulthood, but we'd want to make sure that he's still restricted.

If I have to, I'm willing to blank the system and get a new Live account, but I was hoping to find a way to keep all of his achievements if possible.

Instead of 'demoting' the account to a child account, consider setting up the parental controls yourself in the Family Settings tab. This allows you direct control over the settings to pick and choose a level of security you need. The link below shows some of the power of parental controls.

Xbox 360 family settings

Hope this helps!