Here "children of the flesh" is a metonym that refers to the physical descendants of Abraham. AT: "not all of Abraham's descendants" (See: [[rc://en/ta/man/translate/figs-metonymy]])
This is a metaphor that refers to people who are spiritual descendants, those who have faith in Jesus. (See: [[rc://en/ta/man/translate/figs-metaphor]])
You can translate this in an active form to express that God will give a son to Sarah. AT: "I will give Sarah a son" (See: [[rc://en/ta/man/translate/figs-activepassive]])