Can a parent prevent a child from turning off Find My iPhone?

Use Parental controls in Settings/Screen Time.

Parental controls

Parental Controls and Family Sharing

Officially, see the previous answer. Pragmatically, no. Kids are smart. They learn fast. They learn more about their parents and educators from these efforts than the latter groups might realize or might intend, too. If the local kids truly trust you enough to answer, ask them the various ways that they can disable tracking. It’s not at all difficult. Put differently, you’re going to lose.