ask a relative that you visit from time to time what's their favourite book on their shelf and why. This way, you at least make sure that they would not toss the book in a whim. ask them to write the parental control PIN on one or two pieces of paper and then type it on the phone screen to lock it. ask them to store the piece/s of paper in their favourite book/s.

bow to never bother them about the piece/s of paper unlesls you are stranded in the middle of the ocean and need to use the phone. When you visit them, you might unlock the phone and repeat the process