Romans 3:23-24: “For all have sinned and fall short of the glory of God, being justified as a gift by His grace through the redemption which is in Christ Jesus.”
Romans 5:12-13: “Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned--for until the Law sin was in the world, but sin is not imputed when there is no law.”
Concerning the death of king David’s first baby son through Bathsheba, he stated to his servants: “While the child was still alive, I fasted and wept; for I said, ‘Who knows, the LORD may be gracious to me, that the child may live.’ But now he has died; why should I fast? Can I bring him back again? I will go to him, but he will not return to me.” (2nd Samuel 12:22-23)
Concerning the prophet Ahijah’s prophecy against Jeroboam, king of the northern kingdom of Israel, he stated to Jeroboam’s wife: “Go on home, and when you enter the city, the child will die. All Israel will mourn for him and bury him. He is the only member of your family who will have a proper burial, for this child is the only good thing that the LORD, the God of Israel, sees in the entire family of Jeroboam. And the LORD will raise up a king over Israel who will destroy the family of Jeroboam. This will happen today, even now!” (1st Kings 14:12-14, NLT)
Therefore, it is reasonable to conclude that infants who are contaminated by the Adamic nature, do not necessarily have sin imputed to them, at least until the age or condition of accountability.
Some Calvinists do believe that deceased babies and aborted babies go straight to Hell, and they reason this because otherwise, if perished infants went to Heaven, that it would circumvent the entire plan of salvation through Calvary. However, the purpose of Calvary is only for those upon whom sin is imputed. If anyone upon whom sin is imputed, finds another way to redemption, then, yes, you could argue that something is being circumvented. However, the question remains whether infants belong to the class of those upon whom sin is imputed, which if they are not, then their salvation isn’t circumventing anything. They would simply not be a part of the purpose for which Calvary was designed.