Our family bought the most beautiful book series entitled:
God's Design for Sex Series,
By: Stan Jones, Brenna Jones
The First One is called "The Story of Me" it is for age 3-5
The second is called, "Before I was Born" It is for ages 5-8. We also bought the last two, called, "What's the Big Deal" and Facing the Facts". We read both of the first two books to our 8 year old daughter, 6 year old son, and three year old daughter. As of yet, it hasn't been appropriate to read the third one, as it is really more for when they start asking more specific questions. My 6 year old son actually thanked me, profusely, after reading the two books to him. He took them and read them both a couple more times. Since then, the subject hasn't come up at all again.
The books handle the topic beautifully. They use proper anatomical terms. They approach the topics from a very Bible centered way - explaining God's perfect plan for us and our bodies and our future family. I can't say enough about how well written, illustrated, and informative these books are.
They can be found at CBD.com
http://www.christianbook.com/Christian/Books/product?item...
and I am sure any other online bookstore. I hope this helps.
Blessings,
D.