A couple of good posts on this topic:
I could go on for days about this topic, so it's nice to know that others have thought at length about it, too.
My "rule": Children should read what they like. Will admit to "forbidding" children from checking out both Vanity Fair and Last of the Mohicans; it's never the higher readers who come to the desk with them, it's always someone who doesn't like to read, and those are the two most unexciting books I've ever picked up. I do also censor my collection. I'm not for censoring-- I've helped kids find books at the public library, but I don't want to fight with parents when their children read problem novels for high school students.
That said, I think Alix Flinn's stuff is generally okay for 7th graders, and the occasional 6th.