And of they can't, because they can't. Every parent is not the ideal parent and will never be, heck there's parents who are retarded (in the literal sense).
But lets say it's the average tech illiterate parent, who thinks computers are a gaming machine for kids. Imagine the amount of social engineering and how the marketing departments of companies trying to make kids addicted to their platform. The industries are pouring trillions of dollars in marketing, are you confident that regular parents have a chance of competing with them?
i'm in no way of supporting this because it's done for other nefarious reason, just commenting on the troupe that parents is responsible for their own kids. We can say that all day, but the reality is that they don't know what impact it has on their kids mental health and how damaging short term dopamine addiction is for their children. Society is still responsible to help parents navigate healthy behavior for their children.
Tbh if children was the concern of this law, then you would allow parents to sue tiktok and other platforms that have made their children addicted to the platform. You should also be able to sue influencers for promoting stuff to children (they know the age of their userbase).
Still parenting is hard, especially when you are competing with phds in behavioral science or psychologists that help these platforms design their addiction mechanism.
The solution is simple but there's not enough political motivation to do anything, there is more incentive to do bare minimum. regulate marketing on the internet, enact laws that prohibits intrusive marketing ad platforms.