However, as red herrings go, we were discussing the cat, not the person with the HCL.

At any rate, this would be a bad analogy. Again, sex does not necessarily, or even generally entail harmful effects, especially not any comparable to chemical burns.

The responsibility then falls to the Knowing to create Informed Consent. Obviously, with the dog or cat, education would be impossible. For the little girl, though, education is obviously a logical next step.

I don't believe this would be the ethical responsibility of individuals who interact sexually with little girls (though clearly it would be helpful for the latter to be educated on these matters at some point). I am a consequentialist ethicist, specifically a utilitarian. I believe that, when it comes to evaluating the ethicality of decisions, their particular effects are paramount, specifically as they relate to people's wellbeing. Since it's perfectly possible to safely interact sexually with a little girl without first lecturing her about the possible risks involved, obtaining informed consent is not ethically imperative.