Personally, I would love warnings (speaking for myself and not my kids) because if there is going to be gory violence or -- to a lesser extent -- explicit sex, then *I* want to know about it. My moods vary with the sex, but I *never* want to read books with lots of detailed violence. I think I skip a lot of potentially good books because I'm not certain of the content.
With my kids, at the moment they are linked to my account, so they can't buy anything without me finding out about it; I am guilty of buying books based on reviews and not reading them myself. I do download them for myself to read "someday" (and I hope I'm not unpleasantly surprised when I get around to it.)
That has nothing to do with the topic, except that I wanted to present myself as an involved parent.
That book title looks like one I would refuse them, although I'd probably at least look it up to make sure it really is what it says (and then tell them no again, for emphasis.
)