On the Internet, the responsibility is to properly tag content where appropriate. If porn ends up in a space that is not for porn, that is indeed a problem. Similarly, if a content provider is intentionally advertising to inappropriate consumers, that is also very much a problem. However, if everything is properly labeled, what more can they be held liable for?

An individual breaking the law is the responsibility of the individual. If this individual is a minor, then that responsibility falls on the parents/guardians. In this case, parents have the tools to keep their kids safe. If they choose to ignore these tools and let their kids break the law, I'd almost call that neglect.