That's what I did with my children - I never hid anything from them. They knew about my menstrual cycle and other such things as young children. My daughter was familiar with these topics by the time she reached that age herself.
Speaking about the importance of keeping/protecting their virginity until marriage began as young children, and continued (albeit in more intensified fashion) into their teens.
My children knew there was nothing they couldn't tell me, or ask me and get an honest, forthright, intelligent and informative answer...
Not everyone does this, but I agree with this psychologist. Starting young and hiding nothing is the best way. My children grew to be very well rounded individuals.