you cant stop someone from watching sexual content, remember that one time British made a law to stop it. It didn't work. This is the same problem, you can't block VN with sexual themes, the creators either will lie to trick the system or the kids will find loopholes. In British problem, they can't block random website because there is too many and many also kinda lie by having loophole of nudiest under educational laws. So no matter what you can't stop it, they must learn one way or another.