There are two books that I found really helpful when my nieces started asking questions while I was pregnant with my son.  I had my brother get both of them.  They are:

 

Kids First Book About Sex
by Joani Blank

It’s So Amazing! A Book About Eggs, Sperm, Birth, Babies, and Families  by ROBIE H. HARRIS

Hope these help.  Good luck, I know it’s not an easy topic to get into, but remember this.  If she is old enough to ask, she probably already knows a little.  As a teacher who watches and listens, you would be amazed at what these little pitchers hear and talk about. 

Robin