I’m a bit confused how assigning a PIN to potentially dozens or hundreds of items is supposed to be easier than setting up a Plex Home with accounts for your kids (where you could e.g. simply assign a restriction profile) – ignoring the fact that a single/shared account setup will mostly diminish certain features like viewstates or user ratings.
That being said, there’s an existing feature suggestion discussing an option to secure individual libraries with a PIN instead of using different accounts. I suggest you comment/vote in that thread in order to help us avoid distracting or cannibalizing votes.