A law like this isn't for parents, it's for social media companies. They want to make them lible in instances where their platform facilitates bullying/harassment/etc. That's why there's not penalties for children or parents.

It also sets a cultural precedent to dissuade parents from letting their children use social media. I don't have children so I can't speak from experience but having been a teenager myself on the burgeoning days of social media I would have been fairly annoyed with my parents if they said I couldn't use myspace lol.

Not saying I necessarily agree with it. This isn't a topic I'm well versed in since it doesn't directly affect me specifically, so it's hard to have stake in it.