Whence they, who are not liberated through grace, either because they are not yet able to hear, or because they are unwilling to obey; or again because they did not receive, at the time when they were unable on account of youth to hear,** that bath of regeneration**, which they might have received and through which they might have been saved, are indeed
justly condemned; because they are not without sin, either that which they have
derived from their birth, or that which they have added from their own misconduct. ‘For all have sinned’-whether in Adam or in themselves-‘and come short of the glory of God.’" (Augustine - On Nature and Grace, Against Pelagius, 2, 4)
St. Augustine (On the Soul, Book III) says “If you wish to be a Catholic, do not believe, nor say, nor teach, that infants who die before baptism can obtain the remission of original sin.” A still stronger passage from the same doctor (Epistle 28) reads:“Whoever says that even infants are vivified in Christ when they depart this life without the participation of His Sacrament (Baptism), both opposes the Apostolic preaching and condemns the whole Church which hastens to baptize infants, because it unhesitatingly believes that otherwise they can not possibly be vivified in Christ,” St. Ambrose (II De Abraham., c. xi) speaking of the necessity of baptism, says:" No one is excepted, not the infant, not the one hindered by any necessity."
newadvent.org/cathen/02258b.htm