It is extremely difficult in our modern era to discuss this topic, since there exists so much sexual propaganda and open indoctrination of children. Because such efforts are widespread, intense, and pervasive, it is even more important as parents to be as open, clear, and transparent as possible about it.
You always have to model what you want your child to emulate. You can only teach what you have lived. If transmutation has helped you to be a more decent, upright human being, then it is important to elucidate the process by which you came to that understanding. No one can effectively teach what one hasn't personally lived.
It might be good to first discuss what exactly your child has been exposed to, so that by addressing their current knowledge, you can better educate them. Pranayama is a wonderful practice to begin transmuting the sexual energy and is one of the first practices young people should learn as they are maturing.
The best resource I know of for parents is
Fundamentals of Gnostic Education. This will give you the pedagogical background knowledge to better address you and your child's concerns.