The Most Important Thing to Know:

Genesis 1:27 says "So God created man in His own image;
in the image of God He created him; male and female He created them.
Then God blessed them, and God said to them, 'Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth.'"

God gave us a command to "be fruitful and multiply." Talking about sex in a Christian home should not be a burden, but a chance to share the God-given and God-created development of our bodies with our children. 

It's never too EARLY to start talking to your children about their bodies.
This is how they become comfortable talking to their parents about their bodies, their feelings, and their concerns.