Choose the one app or activity the child needs, lock private content first, and check whether notifications, recent items or cloud libraries expose more than the intended screen.
A game session and a photo handoff are different
For a game, the goal is to keep the child in that activity. For showing a photograph, the goal may be to prevent swiping into the rest of a library. Device-level guided-access or app-pinning controls can help with an ordinary supervised handoff; verify the actual exit behavior on your device before relying on it.
Check the nearby surfaces
Lock NullVault before the handoff. Review visible notifications, recent-app previews, ordinary gallery copies and downloaded files. An encrypted vault does not hide a duplicate that still appears in another app. If a child knows the device passcode, remember that some built-in private albums use that same authentication boundary.
A supervised sample is useful: hand over a harmless screen and check what ordinary navigation reveals. Do not make a child responsible for understanding your file-security system or remembering which destructive settings to avoid.
When separate vault storage helps
Pattern-addressed spaces can keep a private archive separate from everyday content. Automatic lock reduces the duration of an active session. Controlled import still requires you to account for originals left elsewhere.
Use showing one photo without the camera roll for a narrow handoff, Android app pinning for Android-specific steps, or the shared family phone guide for repeated access.