Don't bother too much. Remove all possible apps, set do-not-disturb and block Internet after installing audiobookshelf.
TALK to your kids how and when to use it. They'll understand.
My daughter can use an old Samsung phone with audio jack to listen. I installed LineageOS but this is not required. She can only use it when one of is with her ATM and has to give it back after. I also installed the apps Vlc, PaulWoitaschek/Voice and Lissen because ABS has some bugs and crashes from time to time. Voice / Lissen are also native apps that increase battery life.