More than simply "one who gives oversight to a child," this was usually a slave who was responsible for enforcing rules and behaviors given by the parent and would report to the parent on the child's actions.
Before Christ came, God had planned to justify us. When Christ came, he carried out his plan to justify us. This can be stated in active form. Alternate translation: "so that God would declare us to be righteous" (See: [[rc://en/ta/man/jit/figs-activepassive]])