The Bible is our source of truth, not the ideas of man! We are born in sin, as God says, and it is that nature He is talking about, not what one may do or fail to do in his weak and sinful nature. God would be justified in destroying all mankind, even as He did in the days of Noah, and chose to save Noah and his family (which was a type of what He is doing in this age, and a lesson for us).
Children are born sinners before our holy God, because they have the same malady that we all have from the very beginning, as God says. If they die before they have ability to acquire knowledge of the truth, or are otherwise mentally incapacitated sufficient to render them unable to discern, they cannot be held guilty, though all are procreated by man in sin. They are what they are, and God is a merciful and gracious God, and you and I do not know all of His mind, so speculation is of little help. The Bible does not give us a full answer to this.
The other consideration here is that all mankind is of the "elect" of God or not, and He alone knows by "foreknowledge" who will receive Him (even in a simple way), and who will not. Let us not hurry to make judgments beyond our knowledge.