My 6 year old surprised me with this question this morning.
My spur-of-the moment answer was a daddy and mommy and god make a baby.
Her: "does god cut open the mom's belly to put the baby in there?".
Me: No, (not wanting to get real in depth 5 minutes before she gets on the bus). It's magic, now lets go get ready.

Lol. Help! How did you explain at this young age?