So, I went to a friend who is both a fantastic nurse and the mother of a 15-year-old boy to ask her for -- what else? -- book recommendations, and she recommended two fantastic books. The first, geared toward ages 7 and up, is called It's So Amazing!: A Book About Eggs, Sperm, Birth, Babies, and Families
Both books use very clear, straightforward artwork to convey meaning, with the "younger" book using more of the artwork and cartoon-like panels to give the information. There is no mincing of words, no beating around the bush, and nothing said to make anything seem like it's taboo or titillating or shameful.
Now, instead of trying to explain what Mom has instead of a penis, I can open up It's So Amazing! and show them what Mom, and every other "girl" in the world, has. I've recommended these books so many times to other parents at baseball and soccer games that I figured I would just write a blog post about them and just direct people here from now on. Good luck to everyone raising kids with answering the tough questions!