33 That this right is inviolable St. Thomas proves as follows:The child is naturally something of the father . . . so by natural right the child, before reaching the use of reason, is under the father's care. Hence it would be contrary to natural justice if the child, before the use of reason, were removed from the care of its parents, or if any disposition were made concerning him against the will of the parents. And as this duty on the part of the parents continues up to the time when the child is in a position to provide for itself, this same inviolable parental right of education also endures. "Nature intends not merely the generation of the offspring, but also its development and advance to the perfection of man considered as man, that is, to the state of virtue" says the same St. Thomas.
Source: Divini Illius Magistri (Vatican.va)