J.H.
We have always been extremely up front about the birth process. My oldest daughter, now 6.5 went with me to all of my ob visits when I was pregnant with my youngest who is now almost 4. While both of my daughters were C-section, we have often discussed that while I had to be cut open for them to come out, babies are supposed to be pushed out from a special space between their mothers legs. The girls have also seen Milo and Otis (a movie about the friendship and travels of a cat and dog) which has a natural birth of kittens and puppies, so it is easy to understand the process after watching something natural like that. I would suggest you check out the local library or bookstore for a book that explains it the way you want it explained, but sometimes children are only looking for a basic answer and something as simple as "babies are pushed out by their mother when the time is right" is enough. Best wishes.
Victoria