A short story fairytale that I've been thinking about writing and plotting out has recently given me pause. It focuses on a witch and a princess who both cross faeries, but it isn't by any means for kids, and I don't want to make the mistake of simplifying it or dumbing it down, because it's very dark and complex. How can I keep my fairytale mature and make it clear that the subject matter isn't for little kids?
Stories for adults, based on, or including elements of fairy tales, are quite common. For example, Neil Gaiman's Snow, Glass, Apples is a retelling of Snow White, with an evil vampire Snow White and a necrophiliac prince. "Not for children" doesn't begin to cover how dark and creepy that story is.
I would say, this trope is so common, there's no reason why your readers would expect the story to be for children, unless some weird decision by the publisher makes them think otherwise.