Parental tools built into Android and iOS will prevent the kid from doing anything the parents don't want. Tools provided from the carriers can keep parents informed of EVERYTHING the kids do on that phone and can even lock-down the phone more.
Everything listed in the OP could have been prevented/caught... the parents just didn't take the time to learn how.
Just FYI.. There are no parental controls on a flip-phone. A kid can still text and send/recieve nudes.