This could be possible...

1. In Control Center, put an icon of a semi-lock.
2. When enabled, the app you're in remains available, but if you exit the app, the phone locks.
3. Additional restrictions may be defined such as inability to delete or send items.

This would work for photos, perhaps restricting it to an album or date range, but it could also work for handing your phone to a kid to play a game, or letting someone make a phone call.