For a child who should stay inside one app, Guided Access is usually more appropriate than merely shortening Auto-Lock. It can restrict the iPhone to the current app until an adult authenticates.
- Open Settings > Accessibility and enable Guided Access.
- Set the Guided Access passcode or authentication options.
- Open the app the child should use.
- Use the configured side-button or Home-button shortcut to start Guided Access.
- Choose any touch, button, or time-limit options you need, then begin the session.
End Guided Access with the configured shortcut and adult authentication. For broader child controls across multiple apps, use Screen Time instead.