This is a question I have yet to answer directly.
My girls know the general area where babies come from, but again, I have not felt the need to give specific details.
A few days ago, I overheard Megan talking to Amanda about their future. She was talking about when they have babies of their own ...
Megan: Then you'll pee a baby girl and another baby girl, and I'll pee a baby boy and a baby girl.
See? No details necessary. You take a trip to the restroom and come back with a baby. It's all good.