How does Original Sin come back in man even after being washed at the time of baptism? According to Catholic thought, if we are washed of Original Sin at the time of baptism, why do we transmit Original Sin to our children? In other words, how is it that my child is born with Original Sin when I was freed from Original Sin at the time of my baptism?
I think you misunderstood the role of Baptism in Catholicism. 

Cathecism of the Catholic Church-1263 says


  [1263] By Baptism all sins are forgiven, original sin and all personal
  sins, as well as all punishment for sin. In those who have been
  reborn nothing remains that would impede their entry into the Kingdom
  of God, neither Adam's sin, nor personal sin, nor the consequences of
  sin, the gravest of which is separation from God.


It doesn't say original sin is washed away, it says it is forgiven. Forgiven doesn't mean you don't have original sin anymore; it only means your sins are forgiven.

Our sins cannot be washed away by merely washing with water nor any kind of Baptism. Only the blood of Jesus Christ can wash away our sins.


  1 John 1:7 (NIV) But if we walk in the light, as he is in the light,
  we have fellowship with one another, and the blood of Jesus, his Son,
  purifies us from all sin.
  
  Revelation 7:14 (NKJV) And I said to him, “Sir, you know.” So he said
  to me, “These are the ones who come out of the great tribulation, and
  washed their robes and made them white in the blood of the Lamb.
  
  Revelation 22:14 (DRA) Blessed are they that wash their robes in the
  blood of the Lamb: that they may have a right to the tree of life, and
  may enter in by the gates into the city.