"Positive" as in an act / power of the will. Actual sin requires an act of the will.Sin is a 'positive act'?
Doesn't matter how incapable infants are in making a decision to sin. Sin is imputed to the whole race of Adam. If you're born of Adam you're a sinner. From day one.
Infants are not capable of committing actual sin because they do not possess the full facilities of their will. To believe that man sins just by the fact that he is man renders God the author of sin. With each subsequent conception, the God of Calvinism would be bringing more sin (and therefore evil) into the world.
Once again, the reason why infants and all of us die is because we suffer the effects of original sin.
Romans 5:12 ---> "It was through one man that guilt came into the world; and, since death came owing to guilt, death was handed on to all mankind by one man."
If death, as you are arguing, is due to actual sin, then our Blessed Lord would have been a sinner, as He too suffered death.