"and throughout quarantine it’s felt like they’ve been hiding something from me"

Dear LW, If you feel like your child is hiding something from you, have a conversation with them. Don't snoop. Let them know they can talk to you about anything (assuming they can, don't promise that if they cannot). Because, it may not be the fanfic they are hiding. It may not be just the pandemic that is causing depression.

Hopefully the LW has organised some professional counselling for their child as well. For the depression, not for the fanfic.