Making them illegal does not fix it. There will be a indefinite whack-a-mole game which is very hard to solve without draconian control over the Internet.

The problem is that it's easy to make, easy to deploy, easy to make money on, and a single site opens up the whole Internet. It will happen even if it's illegal.

Compare this to adult webpages setting the header. They will probably be quite willing to do so, since they want to make their money legally, and there is probably little money in serving to kids anyway. And even if a single out of thousand adult webpages refuses, it still only opens that single site.

It's actually hard to understand on "which" side you're on, but a charitable interpretation is that you're arguing that there are no perfect solutions, hence a simple and minimal non-invasive method will probably have the same effect as a complex and invasive method. That is, both methods will add enough friction that children who don't know what they're missing won't bother and the ones who can't do without, will choose every conceivable method to get around the restrictions.

Worrying about the latter makes no sense, because they are sort of like organized crime. People still take drugs even though they are illegal.