I've never heard a newborn speak anything.
The fact sin is lawlessness should cement the fact a newborn is not a sinner.
webdog, please forgive my Wesleyan perspective here but I paid a LOT for that degree and need to get my money's worth...
Wesley defines sin as a "willful transgression of a known law of God." Without spending three years working through it, the emphasis is on the willful transgression. Newborns don't willfully transgress any law. Three year olds certainly do! Can a three year old comprehend they are willfully transgressing a known law of God? More than likely not.
Back to my roots:
London Confession Article 6 The Fall of Man, Sin and Punishment
3. They being the root, and by God's appointment, standing in the room and stead of all mankind, the guilt of this sin was imputed, and their corrupted nature conveyed, to all their posterity descending from them by ordinary generation. Their descendants are therefore conceived in sin, and are by nature the children of wrath, the servants of sin, and the subjects of death and all other miseries, spiritual, temporal, and eternal, unless the Lord Jesus sets them free.
4. All actual transgressions proceed from this original corruption, by which we are utterly indisposed, disabled, and made opposite to all good, and wholly inclined to all evil.
I believe the two views to be complimentary. There is a clear delineation between a sinful nature and an actual, willful transgression. Newborns don't willfully transgress a know law of God but, again, the don't stay newborn for long.