How much swearing is acceptable in a children’s book?

I've been up against this one for the last couple of days: just how much can a writer swear - and when - in a children's book?

Obviously, I'm not talking about picture books for the under fives here. I'm thinking more specifically of the type of reader that number one daughter is: Twelve years old, reading ahead of her years. A Twilight fan par excellence, (books which I tried to read but gave up – I went through my own teen angst, thank you very much. I don't really feel the need to read four whacking great volumes of someone else's, vampires or no), she's also a fan of the Cherub series by Robert Muchamore (which has a certain level of bad language) and the Bartimaeus trilogy by Jonathan Stroud, which doesn't, instead showcasing the fact that you don't need to be swear to convey rude or insulting language. Alright, the Cherub series has its protagonists drawn from children's homes in working class areas, and there are probably few enough linguistic angels there, so I can understand Robert Muchamore's including a few "naughty" words. But elsewhere in children's literatureliterature: how much is too much?

My own feeling is that in reading matters, we should(with a few caveats) leave it to children to be self-censoring. If my daughter picked up The Wasp Factory I might feel obliged to offer her a few words of warning before she got too far ( and I'd probably express my own forthright opinion on the book at the same time..!), but I wouldn't take it out of her hand. The pace of reading is, I think, sufficiently different to watching a film or a television programme, and the level of involvement required – a book requires reading, in a way that other media don't – makes it much easier for us to put a book we don't like down. And to make a massive generalisation (which pointedly excludes The Wasp Factory, by the way!), gratuitous sex, drugs and violence seem to figure much less prominently on my bookshelf than they do on my TV screen. I'm not saying that they're not there and/or equally/more compelling or graphic, here: if anything, quite the reverse. Just closer to their proper places in life.

But in books specifically for children?  I wouldn't be too happy with my daughter having writing designed for her that included f-bombs, the c-word is definitely out, and bugger - that favourite Australianism - greatly offends at least one of my close friends (who I certainly wouldn't describe as overly prudish) and is not, particularly, a word whose true meaning I'd like to explain to my twelve-year old on a winter's night just before bedtime.  Almost anything else would seem to represent something that most kids will have seen coming out of themselves since birth...

Thoughts and opinions welcome!