I have an usborne book "how my body works" which explains the reproductive system in terms that are understandable for primary school kids. It doesn't focus on sex which I like, it's just another body system and they are all explained. DS1 reads it quite frequently as he is currently quite obsessed with bones and muscles.

With our kids we answer questions when they are asked. There have been lots about the creation of children and my older kids are fairly familiar with what sex is and how it creates babies. My twins haven't really asked yet beyond the questions of how they got out my tummy.