I saw this on another platform and thought it would be useful knowledge to put on here.
It works under parental control set ups too, obviously only if your child has WhatsApp installed.
Basically, as well as locking the chat you can hide the locked chats folder, so parents who check their childs phone won't even be able to see if it exists.
To find out if your child has a locked chats folder, click on another chat you can see and try and lock it. If it asks for a secret code, you know they've already set up locked chats.
I found a video on YouTube that shows you how they set it up, so that might be useful too.
For some reason I can't add the image I want to so I'll try and do that in the next post.