Baptism is a external sign that a person has repented of sinning against God, has begun to produce fruits showing that they have repented and begun to follow him (Matthew 3:7-8), has accepted Jesus' sacrifice for their sins, and is willing to commit the rest of their lives to doing God's will. The choice to go under the baptismal waters is a serious commitment that lasts a lifetime and affects a person's future FOREVER.
An infant cannot understand the nature of sin, repent, begin to bring fruits of repentance then contemplate the serious decision of turning over their lives to the Eternal through the ceremony of baptism (see Romans 6). Baptized infants and small children, therefore, no matter how sincerely performed, is not endorsed in the Bible. Not only that, there is NOWHERE in the New Testament where it states God opened a baby's mind to his truth so that they could repent and change their (no doubt incredibly brief) lives!
For further studies into what should lead to baptism and its symbolic meaning see 1Corinthians 1:13 - 17, 1Corinthians 10:1, 1Corinthians 12:13, Galatians 3:27, Ephesians 4:5, Ephesians 5:26, Colossians 2:12, Hebrews 6:2 and 1Peter 3:18 - 20.
In conclusion, infant baptisms are NOT a practice found in the Bible but rather a long-held tradition of the Catholic Church that many other denominations have also accepted. Though no doubt carried out by sincere parents and others, the ceremony itself makes a MOCKERY out of what God requires in order for a person to receive his Holy Spirit and become a true believer in him.