A child is a total person: BODY, SOUL and SPIRIT, uniquely created by God. We cannot deny the truth that God is the Creator and Author of life. Therefore, He is the ultimate expert who knows exactly the total development of a human being from conception up to the time of his or her death.
Psalm 139: 13 -16 say: "For you created my inmost being, you knit me together in my mother's womb, I praise you because I am fearfully and wonderfully made; your works are wonderful, I know that full well. My frame was not hidden from you when I was made in the secret place. When I was woven together in the depths of the earth, your eyes saw my unformed body. All the days ordained for me were written in your book before one of them came to be."
From the moment that a sperm cell unites with an egg cell and fertilization takes place, there is life. The SPIRIT within that living cell is already complete though the SOUL and the BODY need full development.
Genesis 2:7 says: "And God formed man from the dust of the ground and breathed into his nostrils the breath of life, and man became a living being." The breath of life given by God became man's SPIRIT. Thus, a man or a child is a SPIRIT being that possesses a SOUL and lives in a BODY, SOUL & SPIRIT.
Train a child in the way he should go; and when he is old, he will not depart from it. - Proverbs 22:6