Let's face it. Nobody really looks forward to "the talk" with their children. It can be uncomfortable for both of you, and we worry that what we say will be positive and informative, without adding more ideas and questions to our young child's head. Along with the growing up information, and a discussion of appropriate intimacy, we should also talk about the importance of modesty in dress and speech and about morality. I have always thought that these topics covered just about everything, but there is another issue that we should discuss. President Thomas S. Monson said this in 2002: "When I consider the demons who are twins - even immodesty and immorality - I should make them triplets and include pornography. They all three go together."
Pornography is sneaky. It's not as obvious as immodesty and immorality. It can be hidden for a longer period of time. Many people don't see the harm in it saying it's just pictures and it doesn't hurt anyone else. However, it is "a destructive poison that weakens your self-control, changes the way you see others, causes you to lose the guidance of the Spirit, and can even affect your ability to have a normal relationship with your [current or] future spouse (For the Strength of Youth)."
Of course it is up to you to decide when and how to have this discussion with your child, but there are some helpful conversation starters on ThePornTalk.com that offer ideas on how to make the conversation two sided instead of a speech to be endured (by you and your child).
We have read the book, The Trap by Karmel H. Newell with our children and it offers a thoughtful metaphor observing wasps getting themselves caught in a trap and how that relates to what pornography does to us. It is written by members of the LDS church and includes references from scripture and suggestions for parents at the end. It is a great conversation starter too, because after reading it, you naturally want to discuss it.