The Church teaches that we are born with original sin. But if the mother is baptized and thus freed from original sin, how is it that her child is born with original sin?

​Human parents can supply to their children only the fallen human nature that came from Adam and Eve. They cannot supply what is divine and supernatural. 
 
Even in terms of mere human nature, parents are only able to supply the basic elements. So, for example, the child receives from his parents a brain and an intellect, as basic elements that come with human nature. But parents cannot transfer at conception their knowledge of language, or mathematics, or history to the child. The child must acquire this for himself, with the help of others. 
 
If this is the case with human nature, it is even more so with the supernatural life of grace. The mother may be baptized, and free of Original Sin, but she cannot communicate this to her child, only God can. And thus she must bring her child to the Lord in baptism, who begins a saving work in which the child must grow.