Fine, let's change the numbers. If web filtering is 99% effective and there are 20 million pornographic web sites out, then that means there are 200,000 pornographic web sites available to children.
Fine, you changed the numbers. But the matter has already been asked and answered in the US Supreme Court. Concerns such as yours are only attempts to relitigate settled issues in an effort to cloud the minds of those who are not yet aware of the SCOTUS decisions of US v. ALA and Board of Education v. Pico, etc. The law is the law. You don't get to ignore it just because you raise the same issues already raised and shot down in the SCOTUS. Please, read the cases. Here a link to both:
Required Reading