A pre-pubescent child cannot offer consent for reasons both anatomical and hormonal. They are incapable of "getting" sex. They can understand sensuality, such as a good backrub, but they cannot grok sexuality. It's an endocrine thing.
Much more relevant to your argument to ask why we do not allow a post-pubescent child to give consent. A sixteen-year-old, for example, has all of the correct physical development and triggered hormones to function as a sexual creature, and yet we do not see them as consenting partners.
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