Although Naruto is not based on the real world, it is based on many Japanese/asian mythologies. As such, there are still some rules that apply. There should not be a partial soul transfer. If anything, the soul would possess from person to person. In Naruto's case, he has sealed the soul. If he should die, the soul shall diminish with him. As mentioned before, if the seal is broken, and the soul is free, it could go to anyone else, not the baby. It will be highly unlikely for the soul to travel from the Naruto, to the mother, and then to the baby, and I have never heard of partial soul transfer in Asian mythology.