The English word "nurture" (KJV) or "training" (NKJV) indicates caring for and providing supportive instruction. The underlying Greek word more specifically involves educational feeding or instruction, as if in school or for the purpose of learning a discipline. The word thus covers verbal instruction, chastening, and the use of drills needed to produce Christian character. It does not at all indicate that any of these approaches is even harsh, let alone cruel. However, it suggests that parents follow an organized and consistent plan.
The term "admonition" or "instruction" (NIV) means a warning, drawing specific attention to verbal instruction. In summary, Paul touches on three areas vital to child-training so that children keep the fifth commandment properly. "Of the Lord" touches on the standard or quality one is to strive for. "Nurture" indicates what is physically done to and with the child in terms of consistent, regimented training, including discipline. "Admonition" draws attention to what is said and how it is said to the child.
Taken together, then, Paul clearly teaches that child-training is something that can neither be left to chance nor sloughed off with a careless, resigned attitude, as if it were merely a necessary evil. The parents' vision must be long-range. From parents applying right principles consistently will come the gradual development of understanding and wisdom in the children. These are precursors that help produce the promised long life and prosperity in the commandment.
In I Thessalonians 2:7-8, Paul uses himself and his relationship with the Thessalonian congregation as an example:
But we were gentle among you, just as a nursing mother cherishes her own children. So, affectionately longing for you, we were well pleased to impart to you not only the gospel of God, but also our own lives, because you had become dear to us.
He says he treated them with the tender affection of a nursing mother, striving hard so that no one could honestly charge him with taking anything from them. They personally witnessed how gently and consistently he dealt with them as a father does his children by appealing and encouraging them to live their lives to glorify God in their conduct.