The government makes many basic restrictions for protecting children: parents can't give their children drugs or alcohol, porn, guns etc. Social media definitely fits in this category because it has been shown to cause mental harm.
Sorry, no, this is just abdicating your responsibility as a parent. "It's hard" isn't an excuse for throwing your hands up and handing your responsibility over to the state.
Being shown to cause harm is also a meaninglessly low standard. Bathtubs, pools, and bikes can cause harm. You would need to show an actually useful standard. Lets propose will cause an unacceptable level of harmn that cannot be mitigated by less restrictive means.
I don't buy the argument that you are unacceptably harmed because you aren't capable of denying your kid social media nor do I buy the idea that social media couldn't be regulated to be less shitty and harmful.
So precedent exists. Social media is at least as harmful as porn.
Before the internet as I kid I rewired our houses cable wiring and stole my dad's dirty magazines. Pretty sure damn near everyone saw adult material prior to 18.
This is why you find a circle of friends and like mind neighbors who raise their kids in a manner that makes you comfortable. It’s never 1:1, but it doesn’t have to be you against the entire world either. (Though it can certainly feel like that at times)
Think about EU standardizing USB-C for charging. Many people were up in arms: Do you want the state to decide these things? However, personally I benefited. Now I have to carry around only one charger for all my devices.
If the govt standardizes non-smart phones for kids we will all benefit.
Of course this doesn’t address tablets, netbooks etc.
So the actual answer is good parenting, which I posit is one of the bigger problems in the US today and has been for a long time.
Or we keep pretending that pushing education and passing laws about cell phones will somehow be a substitute for bad parenting. The US has been doing that for a long time as well, isn’t working.