Keyloggers can help you know what your kid is doing behind your back when you are not around... : https://www.geckoandfly.com/17868/best-free-keylogger-for-windows-mac-android-ios-to-monitor-your-kids-facebook/
This one seems easy to use... : https://kidlogger.net/
I know there are many mothers on Ovarit who may find this useful!
I think most of these parents you described won't really let their children even have any Internet at all especially when they are not looking... Sorry about what you went though by the way! I first learned about keyloggers when I was doing research on cyber security stuff myself after I was harassed online...