In Colossians 3:18, Paul is giving wives instruction on how God has called them to relate to their husband.
In Colossians 3:19, Paul gives instruction to husbands on how God has called them to relate to their wives.
In Colossians 3:20, Paul gives instruction on how children are called to relate to their parents.
In Colossians 3:21, Paul gives instructions to parents on how to raise up their children in the nurture and admonition of the Lord.