I was referred to some books written by Stan and Brenna Jones. These books have won the "Christian Book Award" - but they discuss God's design for sex (and all the things that go along with that). These are books you can read with your children at ages three to five, five to eight, eight to eleven, and eleven to fourteen. The parent's resource is the one I recommend you read as a parent first "How and When to Tell Your Kids about Sex. A lifelong approach to shaping your child's sexual character". Then, there are 4 books to share with your kids. "The Story of Me" is book 1 and it's for ages 3 to 5 and it's goal is to lay a spiritual foundation for the child's understanding of sexuality. "Before I was Born" is book 2 and is for ages 5 to 8, "What's the Big Deal? Why God Cares about Sex" is for ages 8 - 11, and "Facing the Facts: The Truth about Sex and You" is for ages 11 - 14.
All these books were written as if dialogue were an ongoing reality between mother, father, and children in the home.
My children are 6 and 8 -- so I have read Book 1 with them so far -- and I wanted them to absorb the information in that book first. Then later this summer, I plan to read Book 2 with them. These books are a great resource and a great thing to have in your home so if and/or when they need to refer to them again in the years to come, I think it's great to have around. Also, it's difficult to have these conversations with your kids -- but it's so important that they hear this from their parents before their friends -- and I find using these books as a resource really helps make this much simplier to explain and they know you have a resource with which you are sharing this information from.
Best of luck!