B.M.
Hi S.,
The American Girl book is GREAT. But, 5 may be a bit young for that.... I got it for my daughter in between 1st and 2nd grade I think. I think it's called "The Care and Keeping of You". You could buy it and read over the pages with her that you think are relevant. There are some good pictures that are pretty specific - without going into too much detail.
Be cautious at this age..... you don't want to give her TOO much info. Also remember that when she goes to school she is going to repeat 75% of what shes told, so maybe just the basics for a couple more years.
I am with you, though - I always answered my daughters questions honestly.