God Made Your Body, Jim Burns
This book is a great one to start with to explain to young children how God made them. Aimed at ages 3-5, it uses simple, honest text and great, colourful photographs to explain how God created them. It talks about how God made each of us the way we are – how we look, and what we are good at doing, and the simple anatomical differences between boys and girls which are needed to make babies when they are grown up. It briefly explains how mum & dad make the baby and how it grows. My kids all still remember that once they were the size of a cheerio! It also has a brief description of adoption, which I thought was a wonderful inclusion.
It is a great beginner book to introduce kids to how they were made, that are loved and in a family, and it was all done by God.