It's not really a free speech question here. For instance, what a student can say in a public school (government tax payer funded) can be restricted to a point,

It's not really a free speech question here. For instance, what a student can say in a public school (government tax payer funded) can be restricted to a point, but the courts have ruled that the 1st amendment still protects kids up to a point here. In a private school there are no such protections. The law here is saying that social media companies are private companies and in order to do business with children they need to follow certain regulations now. This is perfectly normal and legal. For instance, a tavern has certain regulations regarding minors entry to them. There are many examples.