11 Then he turns himself to her, “O daughter of Babylon, unhappy;” unhappy in your very exulting, your presumption, your enmity; “unhappy daughter of Babylon!”. The city is called both Babylon, and daughter of Babylon: just as they speak of “Jerusalem” and “the daughter of Jerusalem,” “Sion” and “the daughter of Sion,” “the Church” and “the daughter of the Church.” As it succeeds the other, it is called “daughter;” as it is preferred before the other, it is called “mother.” There was a former Babylon; did the people remain in it? Because it succeeds to Babylon, it is called daughter of Babylon. O daughter of Babylon, “unhappy” thou!...
12. “Happy shall he be that repays you, as you have served us.” What repayment means he? Herewith the Psalm closes, “Happy, that takes and dashes your little ones against the rock”. Her he calls unhappy, but him happy who pays her as she has served us. Do we ask, what reward? This is the repayment. For what has that Babylon done to us? We have already sung in another Psalm, “The words of the wicked have prevailed against us.” For when we were born, the confusion of this world found us, and choked us while yet infants with the empty notions of various errors. The infant that is born destined to be a citizen of Jerusalem, and in God's predestination already a citizen, but meanwhile a prisoner for a time, when learns he to love ought, save what his parents have whispered into his ears? They teach him and train him in avarice, robbery, daily lying, the worship of various idols and devils, the unlawful remedies of enchantments and amulets. What shall one yet an infant do, a tender soul, observing what its elders do, save follow that which it sees them doing. Babylon then has persecuted us when little, but God has given us when grown up knowledge of ourselves, that we should not follow the errors of our parents....How shall they repay her? As she has served us. Let her little ones be choked in turn: yea let her little ones in turn be dashed, and die. What are the little ones of Babylon? Evil desires at their birth. For there are, who have to fight with inveterate lusts. When lust is born, before evil habit gives it strength against you, when lust is little, by no means let it gain the strength of evil habit; when it is little, dash it. But you fear, lest though dashed it die not; “Dash it against the Rock; and that Rock is Christ.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)