It is an interesting question Millhouse, there was talk a short while ago about putting suggested reading ages on children's books, so maybe it's only a matter of time.
I sincerely hope "they" don't come up with certificates on books as a concept, or even charts on the backs of covers telling you there is moderate language and violence. I'm sick of the nanny state telling me what I can do and think. Grrrrr.
The beauty of a book is that if you don't like it you can close it and walk away.
I suggest that if parents don't want their children reading Silence of the lambs that they don't let them. Also, radical thinking I know but perhaps the parent might like to choose books with their child, and discuss what they've read. I know a lot of parents who read a book (or watch a film) before deciding if it's suitable, appropriate and acceptable for their child.
Sorry if it sounds like I'm on my soapbox here, but I am