You're presuming a great deal here.
What am I presuming? Most social media platforms let you have post-level privacy settings.
If a kid wants to post something that they don't want their parents to see they can exclude them from seeing the post
It’s not necessarily about the parent being excluded. It’s about the kid, the kids friends, and the kids friends parents all knowing that a) bad behavior will get back to the parent and b) screenshots are forever.
It’s an entire community.