When I became a Catholic I assumed I would confess all the horrible things I did in the past. I mean I led a rotten life. But at the first confession, I was told that none of that counted, and basically I had to confess only that day prior to my baptism. I assume the priest was right but why?
Baptism is an incredible sacrament. It takes away ALL sin, original and personal, and the punishment due to sin. If you are an adult receiving Baptism, all that is necessary before baptism is sincere sorrow for your sins (repentance).
Acts 2:37-38: Now when they heard this they were cut to the heart, and said to Peter and the rest of the apostles, “Brethren, what shall we do?” And Peter said to them, "Repent, and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit.
For young children, since they are not yet capable of sinning, only Baptism is necessary.