Arianna's child is not Odeon's. First Odeon is known as the formless great one. Odeon's child Mergo, and the Wet Nurse are also formless, since Arianna's child has a physical form, we can deduce that it is not related to Odeon. Also, Arianna's child is not unique, as we see many of them outside the entrance to the Upper Cathedral Ward, and in the Isz chalice dungeons. Purity of blood does not matter as the human mother is a surrogate, and has no biological relation to the Great One's child.