It genuinely worries me mate , I have told them hat it's not the stuff they should be looking at , I think after reading your post I might try explaining why in more detail , but we both know what 13 and 15 year old boys are like , I knew far better than my stupid parents at that age !
Got to agree.with you there, I think it's terrible some of the stuff they've undoubtedly viewed , mine swear they haven't, but I don't really believe them.