I agree with Perlona... if you try to ban these sites they will just go underground and you won't find them or know when your children are on them.
It is also an abuse of 'free speech' however much one might want to dissuade people from using these sites.
In my view, a better way to tackle them is to bring them into the open, show your children that these people are actually ill, have no choice and wouldn't choose to live that way if they had that choice. They are missing out on a great deal and those that manage to live with the illness can look forward to a 'lifetime' of pain and unfulfilled dreams of career, relationships, potentially parenthood and just the joy and challenge of living.
I was watching a half-documentary recently where young teen girls were 'celebrating' pro-ana, the thinness and control and greatly admiring the super-slim celebs... it was a different side of the coin though to look at non-celeb, older women with the disease and the doctor was matter of factly warning these girls that they could kiss goodbye to their looks too. "Anorexia might be clouded with the glamour of youth but nothing will cover it once the bloom has gone.". I thought that was quite a sobering - and helpful - way of looking at it really.
If anything, Anorexia needs to be normalised as just another illness like any other and the mystique and misplaced 'glamour' of it, removed.
I'm sorry for any parent who is watching their child go through this awful illness right now.