There is nothing about being a child that materially or logically precludes the capacity to consent to sexual acts, or any other activity for that matter.

Informed consent.

The cat (in your commentary) is receptive to your strokes but knows nothing about the hydrochloric acid you are about to pour on its back. Yet, it consents to being touched. Then, feeling it has had enough lovin', it walks away to clean its fur. It then becomes seriously harmed by something that it had unknowingly consented to. Would the cat have consented if it knew about hydrochloric acid?