Baptism is the washing of regeneration (Titus 3:5). It is where we were born of water and the Spirit (John 3:5), and became sons and daughters of God in Jesus Christ by receiving the Spirit of the Son.
Coming home to the Father's embrace (conversion) requires repentance or turning away from sin, because light and darkness cannot coexist. We cannot truly convert and continue sinning deliberately. Since, most likely, we, adults have already committed sins in different ways with number of times that can no longer be counted, it is necessary to a real conversion that we confess our sinfulness, declare our will to turn away from it, and pledge our allegiance and loyalty to the Lord.
In the case of infants and small children, this is not necessary because they are still considered innocent. This, however, should not be a reason for them not to be baptized, since this spiritual washing has its effect even without the consent of the child. It is not the same with an adult who had been baptized with an unrepentant heart. The latter is like a beast washed with clean water while soaked in the mire. The former, on the other hand, have no mud that needs to be washed off from their soul; but like everyone else, they need to be born again from above since "unless one is born of water and the Spirit he cannot enter into the kingdom of God" (John 3:5). It is the moral obligation of the Christian parents or guardians to make sure that a child is baptized as soon as possible, just as it is the moral obligation of the Jewish parents to circumcise their male babies (Leviticus 12:3). We know that the Jewish circumcision is a type of the Christian baptism (Colossians 2:11-12), and that fact explains a very important point. A faithful Jewish father will not say, "Let the child decide for himself whether he wants to be circumcised;" and neither will a faithful Christian father say the same if he understands what baptism really is.
The water of the Great Flood is a type of the baptismal water. Noah and his family passed through the flood in order to be saved. Just because they were all adults in the ark doesn't mean they would leave all children behind had it happened that they had younger members in the family. Salvation is for all who are willing to accept it, but not only for them; it is also for their children (Acts 2:39). And even though we have not seen small children among those who were saved through the water, we know, through commonsense, that those who passed over the Red Sea were individuals coming from all ages. All of them were baptized into Moses in the cloud and in the sea (1 Corinthians 10:2).
We cannot say that babies don't need to be baptized just because they have not sinned; or worse, that they should not be baptized because they cannot repent. "Death reigned from the time of Adam to the time of Moses, even over those who did not sin by breaking a command, as did Adam, who is a pattern of the one to come" (Romans 5:14). All of us are parts of the dead Adam. Unless we were reborn in the manner that God willed, we will remain dead and cannot approach God. We are to remain outside of Paradise like Adam.
As people of God, it is morally wrong for us to hinder the children from approaching the Father through Jesus Christ (Matthew 19:14). It is actually in them that God reigns since their hearts are still free from moral disobedience, and yet they need to be renewed because they were born with the flesh of the fallen Adam. We are to help the children grow in holiness, not only through mere words of instruction, inspiration, and discipline but with the help of real grace coming through the sacraments, starting with baptism.
The dead flesh that we inherited is real, the regeneration that we should undergo is real, and the elements that are involved in this washing are real. We are not speaking of symbols but of real things, and of real power coming from the treasure of merits which Christ gained for us on the cross, and we receive through the sacraments.
While adults need to decide that they will turn away from their old life before receiving the washing of rebirth, the infants don't have this need since they have no old life to turn away from. Christian parents must ask baptism from the Church for their children and promise that they will guide them to a life that is worthy of God's children.