Since a child's body comes from both the father and the mother, does its soul come from them?
A child’s soul comes from neither the father nor the mother. Each soul is created directly by God from nothing at the moment of conception.
Some early Christian theologians held the theory that the child’s soul is an offshoot of the father’s. Augustine, for example, regarded this position, known as generationism or traducianism, as possible, though he did not exclude the direct creation of the soul from nothing.
As theologians contemplated the teaching of Scripture and of the Tradition of the Church, they realized that generationism is incompatible with it. Thomas went so far as to condemn generationism as heretical (Summa Theologiae I:118:2).