To unlock this task, you must complete the "Kids on Patrol" side quest in Chapter 8.
This page of the guide to Final Fantasy 7 Remake has a walkthrough for the Missing Child side mission available in Chapter 14 - In Search of Hope.
You start the side quest by talking to Ms. Folia. First, you must look for the children, just like in the Kids on Patrol quest. This time the task is much simpler, you only have to go to one place.
Head to Slum Public Cemetery. Oates is standing in the square.
After a brief conversation with the kid, the opponents will appear. Phantoms are an enhanced version of Ghosts. They are very susceptible to fire magic and physical damage. After defeating the opponents, you complete the task and can return to the other activities in this chapter.
Reward: Time Materia.