A.R.
Hi T.,
My daughter Montanna (4 1/2) has been asking the same questions since she was about 3. First of all I did have a c-section, so she does know where she came out, but she also wanted to know how she got in there. I told her she has / or a part of her has always been in my tummy since I was born.
But if you ask Montanna how she got into my tummy she will tell you that she shwooped in through my belly button.
As for a book with illustrations we have a couple books about where babies come from. I would suggest looking at Barnes & Noble or a store like that in the baby/motherhood section and you should find books there to help explain it to children.
Good Luck, sorry I couldn't give you titles though.
A.