Baptism is the one sacrament that all Christian denominations share in common. In the Catholic Church, infants are baptized to welcome them into the Catholic faith and to free them from the original sin they were born with.
To the Catholic Church, original sin isn’t a personal sin of the unborn, but a sin transmitted from generation to generation by birth. All men and women are born with original sin, and only Baptism can wash it away. Baptism can be regarded as a vaccine against sin.
After the sin of Adam and Eve in the garden, all people are now born with original sin due to our fallen human nature. Through the gift of grace in Baptism, God washes away this stain of original sin and makes us a part of His family and offer us eternal life.
Children, who are born with the stain of original sin, are also in need of Baptism, in order to free them from the bondage of original sin and make them children of God. Our loving Father does not wish to withhold His love and grace from anyone, including children. Baptism simply requires openness.