ANSWER: Mankind was given the gift of being able to reproduce themselves through having children. The ability to have a child, however, serves a purpose far above what most people realize. In the last book of the Old Testament, Malachi, God directly answers priests who serve him regarding a variety of issues. One issue he addresses is the complaint of the priests that their offerings to him were not being accepted. God's answer reveals HIS reason for giving mankind the ability to marry and procreate.
14 You ask why he (God) no longer accepts them (the offerings from the priests). It is because he knows you have broken your promise to the wife you married when you were young. She was your partner, and you have broken your promise to her . . . that you would be faithful to her.
15 Didn't God make you one body and spirit with her? What was his purpose in this? It was that you should have children who are truly God’s people (the KJV has 'godly seed', while the NKJV has 'godly offspring' - Malachi 2:14 - 15)
The ultimate purpose of reproduction is to create children who will ultimately be spiritual sons and daughters of God. In a very profound sense, God is reproducing himself through the humans he made! This is why properly training a child is so vitally important.
The New Testament says that children should be taught to be obedient to parents, that Jesus is the Messiah and Savior of man and that he loves them and that they should obey God's commandments and laws. Teaching a child is a very important responsibility, as it sets them on a path that may last their entire lives (Proverbs 22:6).
The first thing a child should learn is to obey their parents.
Children, it is your Christian duty to obey your parents always, for that is what pleases God. (Colossians 3:20)
1 Remember that there will be difficult times in the last days. 2 People will be selfish, greedy . . . disobedient to their parents (2Timothy 3:1 - 2)
The second thing children should learn is that Jesus loves them and cares personally for their well-being.
2. And after calling a little child to Him, Jesus set him in their midst, 3. And said, 'Truly I say to you, unless you are converted and become as little children, there is no way that you shall enter into the kingdom of heaven. 6. But whoever shall cause one of these little ones who believe in Me to offend, it would be better for him that a millstone be hung around his neck and he be sunk in the depths of the sea.' (Matthew 18:2 - 3, 6)
The third and last thing any child should learn is what are God's commandments, all of which are good for them. Jesus understood this principle when he was 12 years old attending the Feast of the Passover in Jerusalem with his parents. When the festival ended he stayed in the temple asking questions instead of leaving with his parents.
46 On the third day they (Mary and Joseph) found him in (Jerusalem's) Temple, sitting with the Jewish teachers, listening to them and asking questions. (This verse also indicates how children were taught; they were taught by back and forth discussions about God's law with adults) - Luke 2:42 - 43, 46
14. But as for you (Paul is writing to Timothy, a fellow evangelist and close friend), continue in the things that you did learn and were assured of, knowing from whom you have learned them; 15. And that from a child you have known the Holy Writings (the Old Testament) . . . (2Timothy 3:14 - 15)
There are several places in the Bible that discuss children and what they should learn. For additional study please read what the book of Proverbs says about raising a child.