Okay, I'm not understanding the relationship you are asking about. Are you asking if the child is from two cousins? If so, the suggested relationship through shared dna would still be that of parent and child = 50%. The child would share more dna than typical with the shared grandparents of the parents. But the dna test of the child would not indicate that his/her parents were first cousins. If that is what you are asking. And I'm not sure if it is.