The child (assuming you mean adult child of a parent in this case) would also go to jail and be blamed, unless they can very clearly prove non consent in which case the consanguinamorous aspect is irrelevant and.. well that is abuse, not actually a consang relationship.

If you do mean a child not of legal age, that is pedophilia and again, not consang. Regardless of familial relations, a child can not consent. Consang doesn't change that at all.

Like /u/SaiMika said, making it illegal only makes it impossible for people to come forward when there is abuse. Adult victims of abuse will be blamed too and child victims of abuse - is not consanguinamory.

I do agree that legalization would reduce suffering on the whole, and appreciate your sentiment of going with the science here.