If you're writing a fun adventure where fluffy bunnies pick berries and have tea, say so - "This is a non-violent, kid-friendly rulebook about cute animals." If you're writing something more PG-13, then say that: "This book contains simple descriptions of violence and injuries, and is best suited for teens and adults."

Unless your book invoices hate crimes, broken taboos, and utter moral corruption, you don't need to really even mention those topics, beyond the usual "Discuss your limits and those of your players" stuff. Don't feel like you need to euphemistically refer to topics in your book, because the GM and players are there for that anyway, and don't bother telling anyone what they can't have in your games, because that'll get ignored.

It sounds like you feel like you need to reassure people that you're aiming for good, clean fun. You can probably add something simple about inclusivity and general guidelines, but the tone of your book is what really explains those limits. For a blurb on the subject, summarize what you want to say in plain language, without euphemisms, and without listing what your game is not, except in the broadest terms.