If the child cannot understand what a parent is saying, is it rational for the parent to speak to him or her? Baptist parents as well as others speak to their infants, and do not expect the child to understand or to talk back for many months. They see nothing irrational in this. They speak to their children, that is, they employ symbols, not because they think the infant understands all that is being said or because they expect an immediate response. They speak to their children so that the child will learn to understand and talk back. So too, we baptize babies not because they can fully understand what is happening to them, nor because we expect them to undergo some kind of immediate moral transformation. We baptize them, and consistently remind them of their baptism and its implications, so that they will come to understanding and mature faith.

via Talking to Babies and Infant Baptism | Resurrectio et Vita.