David: How do babies get in the mommy's tummy?

Mommy: The Mommy has an egg, and the Daddy has a sperm. They get together (big left out part here) to form a fetus (actually a blastocyst at that stage but that might be too much information). The fetus grows in the mommy's uterus.

David: S. has a baby in her tummy.

Mommy: Yes, S. has a fetus in her uterus.

David: Mommom is making a blanket for S. and C.

Mommy: That's right.

David: S. has a vagina.

Mommy: That's right.

David: Does C. have a vagina?

Mommy: Yes, she does. A friend gave them the sperm.

David: Ok.