When you say that babies sin, and that a 3 year old's actions constitute proof of their sinful nature. They do so no more than a puppy, or an excited dog.

Their awareness is limited to that of oneself. The concept of consequence and of impact on the community are beyond the child's reasoning. As the child grows it starts to become aware of IMMEDIATE impact, grasping the concept of "No" FAR earlier than it does of the ramifications of it's actions. Note that I consider "Sin" to be an abstract impact on the infinitely sensitive "super-being in the sky." Since a child learns from action vs. reaction, the feelings, emotions, etc. of the guardians are the first guide of development.

Until you indoctrinate (brainwash) the child into feeling guilty for offending an all-powerful, invisible, non-intervening being. That child is learning and adapting to society, and developing it's own set of ethics. That is, before you overwrite said ethics with a set of "biblical morals."

A baby is neither good nor evil, though capable of becoming either. A baby's guardians are responsible for it's immediate development, though I would gladly accept help from your "god" anytime he/she wished to come down and help out in the rearing of said baby, I would think omnipotence/omniscience would make the task much easier. I won't hold my breath, though.