Laws hold parents accountable for their childrens' crimes, not their noncriminal actions. Nothing about this is saying that accessing social media is a crime -- that would be more similar to drug possession laws, firearms licensing, etc.

If your child is drinking: they are violating the alcohol possession age limit themselves; you are liable for their crime plus child endangerment if you gave them the alcohol; and whoever sold or supplied them the alcohol is violating a separate law. Sounds like we're trying to apply the same structure to social media, except (so far) with no possession/usage law.