Oh I feel this so much!! I have a 15 year old and the worry never stops tbh. I dont know much about screenshot apps specifically but I do know there are parental control apps out there like Bark or Qustodio that are made for this kind of thing… I think they have to be installed on the phone tho and the person usually knows?
Sorry idk if that helps 
maybe someone else on here has tried something more discreet?