My thoughts exactly. While there are some categories which are easy to agree on as being unsuitable for kids (and some adults, for that matter), you will inevitably find such content restrictions used to attempt to legislate morality.
Look at Florida and their "don't say gay" bill. They want kids to remain oblivious to the existence of LGBT people, using "think of the children" rhetoric, and I have no doubt they would use "child friendly internet" legislation to ban even the most milquetoast acknowledgment of our existence, because they see it all as "indoctrination" and "grooming." As a lesbian, I find this abhorrent, but even if you're not LGBT, you should be afraid of these kinds of laws, because the Christian right will inevitably use them against you, too. If you're atheist (and if you use HN, you probably are), believe in the theory of evolution, believe in climate change, are a person of color...you get the picture.
And then there's the fact that the internet is international. Content about women's suffrage, for instance, would likely be deemed "inappropriate for children" in much of the Arab world, both because most of those nations are not democratic, but also because they want to avoid empowering women.
The suggestions to "just use the MPA rating system" are bad because, even if those ratings were objective (they're not), the good thing about them is that they're not legislatively enforced. If a child wants to see an R-rated movie, the theater won't sell them a ticket directly, but if they're accompanied by a parent, they can see it, and that's probably the best system we can hope for: a voluntary system where parents can allow their kids access to what they feel is appropriate. And for all the flaws that has, it's better than having CPS take your kids because you let them read the wikipedia page about being trans.