A newborn baby is unstained from sin, they have done no wrong and are not even capable of sinning at that point since they have no capacity to choose between good and evil. Oh I agree that the consequences of the fall come upon us. We will die, and our nature will lead us to commiting sin sooner or later, but we are not stained the sins of other people. The notion that we are not stained by the sin of our first parents is Pelagianism, the heresy condemned by the Council of Carthage in 420, and by the Third Ecumenical Council at Ephesus in 431.
-A8