Calling someone a human being is inherently subjective. If it were possible to transplant the correct set of organs to Mary to keep her alive, the same doctors would surely say that it was a human being who would receive them. The Catholic Church has long advocated providing life-sustaining care to those who can be kept alive. However, it has not argued that medicine performs miracles. In this case, the church cannot reasonably argue that both children should die in order to prevent hastening the death of one child.
Trading lives is the ultimate human choice, but sometimes it's unavoidable.
Let's just bite the bullet. It seems to me that once again, this is a case of government officials getting too involved in moral matters that have nothing to do with the reasons they were put into office. I think that the government does a terrible job making and enforcing laws about ethics. I cannot imagine an American court doing the same thing. But I also think that it would be unreasonable for both children to die.