The Birds and the Bees

I knew this day would come eventually. I just didn't know it would be this soon. The other day, my daughters wanted to know how babies eat when they're in their mommy's tummies. They were fascinated to learn about the purpose of their belly buttons, although I'm still trying to explain why three of them are "outies" and one is an "innie." Then came the dreaded follow up: "How do babies get in their mommy's tummies?" I was driving and able to deflect with this wimpish response: "Look! Horses!"
Since that initial conversation, the question has not come up again. Yet, I think I need to take some action. I've been scouring the internet for age-appropriate books. I'm not ready to talk to them about puberty, menstruation, or any of those related topics. I am, however, trying to find a simple way to explain the birds and the bees. I haven't been happy with what I've found. Some books are too childish (using rhyming tactics that reviewers say make the message silly), others are beyond what a nearly-five-year-old needs to know.
I know some people might be shaking their heads and thinking it's simply too early to have this conversation. My girls are in school. They are in school with children who have younger siblings and have likely already had some version of "the Talk." I want them to hear these things from me first, not hear it on the playground. So.... any suggestions?