We're big fans of the "It's so Amazing" book and there is one by the same authors geared at older kids as well. I will caution that it covers some topics that some families might consider touchy so pre-read if you think this might be an issue.
We did this when the kids were younger so it wasn't a big deal that we read it at bed time and then discussed (we were always reading to them anyways...). I wasn't the most comfortable about this but it really helped me that I was just reading the book rather than trying to figure out what to say on my own. Once we got the ball rolling it became easier. Some deep breathes and I survived. I just kept reminding myself that my level of comfort was irrelevant to my kids getting factual information for their health and happiness. Eventually I actually felt not too bad about it and it opened the door to hopefully continue to have discussions as this stuff actually becomes relevant to their lives.
Not sure that helps, good luck!
