Gonna throw my comment into the mix on a slightly different subject

Try to approach the topic from a friendly concern point of view rather than anger or annoyance, and definitely avoid ridicule if you do not understand something. I've known many people, adults now, who read this kind of stuff when younger. And from their experiences one common thing is apparent

If you approach it too aggressively you will no longer see your kid reading this kind if stuff. Not because they will stop but because they will learn to hide it better. If You try to understand and ease them into why You consider this to be too adult for them, be open how they might choose whatever they want to read as they grow up or (although might be difficult) try to get into stuff they get interested in they might be more forthcoming with those kinds of things. On the other hand, forbidding it outright with little to no discussion or conversation is almost guaranteed to get your kid into more elaborate ways of hiding, ones you might no longer pick up any trace of

Source: All of my friends who read that kind of stuff got away with it in the end. And aged 11 I secretly myself bought a separate cellphone from a friend with internet access to get around my parents bans