Heartbreaking 
I know from when my ds was self harming and eventually overdosed that his phone was hugely instrumental in his poor mental health. After it all happened, I looked at his phone and the amount of depressive memes, self harm websites (he had even created an Insta account called 'boys cut too' ) texts from other kids in his year who felt the same and were asking for his advice etc it was HORRIBLE.
I have heard of an app call 'Safe to Net' which is exactly that shadypines.
It uses algorithms or artificial intelligence (?) to work out what your dc are talking about or looking at, then alerts the parents. So for example, if they kept looking at 'anorexia' websites or chatting about 'anorexia' parents are informed without seeing the actual content/private messages.
It also has a feature that prevents nudes being taken. The camera is disabled for a while first, then longer if they try again and if they try for a 3rd time, parents are informed.
I am definitely going to look into installing it on my younger dc's phones when they get one in the summer, just before they start secondary.
I would be interested to see what others think of this app (sorry, shit with computers, can't do clicky links!)