Obviously every family will handle these subjects differently, which is as it should be. In fact, each child may require a slightly different approach. My intent here is merely to share what we have done, not to prescribe what anyone else should do.
A great parent resource is Kevin Leman's A Chicken's Guide to Talking Turkey with Your Kids About Sex. In fact, with this book alone a family would be in great shape to adequately handle these topics without any other resources.
We generally first approach these topics very informally by talking about babies. Anytime I, or anyone in our immediate circle of family and friends, am pregnant, I pull out our books about babies. We especially like Hello Baby by Lizzy Rockwell, which follows a pregnancy through the first day at home with baby from the point of view of big brother, and The Miracle of Birth by Jenny Bryan, which uses acetate overlays to show what is going on inside of Mom as the baby develops. As we read books like these together, inevitably we discuss eggs and sperm, the ovaries, the uterus--laying the groundwork for later discussions.
Once a child starts to ask more specific questions about how the sperm gets to the eggs, we explain in more detail. At that point, a great book to use is Before I Was Born by Carolyn Nystrom. This book explains intercourse in a tasteful way, focusing on God's design for the marriage relationship. We generally explain the subject ourselves first, then follow up with the book to make sure we were clear about all aspects.
At the same time, children need to understand how their own bodies are developing and be prepared for the changes they will encounter. We try to do this a little early, because we want to have discussed it before any of the changes actually occur and also before any other children raise them. So we aim for seven or eight years old to cover puberty issues, realizing that many families prefer to wait until later. For our girls, we've used The Care and Keeping of You from American Girl (although our edition is an old one--we haven't reviewed the current edition). We do skip the content on the question and answer pages between each section--those delve into topics we do not wish to cover and provide answers with which we are not always comfortable. For the boys, we use The Boys' Body Guide by Frank Hawkins. Both of these books cover the whole gamut of developmental topics with a matter-of-fact tone, without slang or silliness.
A bit later, we're ready to talk about other aspects of growing up, the emotional and relational aspects. With the girls, we love Beautiful Girlhood by Mabel Hale. We used the version updated by Karen Andreola, but the original is available free online. This book, in typical Victorian fashion. explains the struggles and joys and pitfalls and beauties of growing into womanhood in direct but discreet fashion, dealing with young ladies forthrightly but without the coarseness so many modern books embrace. For the boys, we plan to use Boyhood and Beyond by Bob Schultz. I'll have to update this post after we've actually used the book with a real live boy!
None of our children has yet entered the teenage years, so we haven't used any resources beyond these. I have previewed many, though, and have rejected most as being far too descriptive of abuse and transgression. I do not believe it necessary to include detailed descriptions of awful situations in order to provide guidance and support for children growing into adulthood.