That book sounds really neat!
Children get gidish when they see animals "doing it", but I guess that is a good way to introduce them to the topic. Putting on the Discovery channel or a book such as this is great at any age. But, just make sure there are no horses in the book because, man oh man!!! That could scare them! he he or neigh, neigh.
Dating back to my childrens "potty training years", I always used the appropriate (physiological) name for their body parts. Children need concrete information to work with and making pet names for body parts, I think, should be saved for "adult play". You know --- how babies are made?
Everything I learned about "growing up", I did not learn from my Mom or Dad. I got my first period in 7th grade (all over) a cafeteria bench at school, and "Thank God", I had an older sister to explain things about that one.
So, don't fret! Teaching about the birds and the bees will just fall in place. Honesty is very important from the beginning because when they grow up they will share LOADS of information with you voluntarily because they know you will understand, accept, and still love them!
Good luck and many blessings on this one!