Photographs and brief text introduce general concepts of human reproduction. A separate text for adults provides more specific detail and suggestions for discussing the subject with children.
Sara Bonnett Stein wrote books for children, some on sensitive subjects, such as divorce and death. She was also involved in toy design and native plant gardening, and wrote on ecology.
Original copyright is 1974. I got this from our library, and I found it really useful. It has lots of pictures and simple text, and lets the child ask questions according to their level of development and interest. I put it on the for-children shelf, but it is for parents and children to read together. My daughter was most interested in the puppies & their mothers this time, but she had some basic questions about human development and sexuality the last time we read it (about a year ago).