Wondering at ages 8 and 9?
I believe the "talk" starts much younger when you answer questions in an age appropriate manner so that your children will always come to you for the questions. Also, it is an ongoing talk... It is not a wham bam thank you mam discussion and you are done. It is ongoing. I started when my daughter was a toddler asking why she didn't have boobs like me.
As a long time substitute teacher in elementary school, I can promise you that your girls know something about sex and most likely not what you think they should know. We have 1st graders referring to body parts, kissing, love, sex, etc. They may not know what you would prefer them to know at this point but this is talked about among peers ALL the time.
In 4th grade our school nurse has a video for girls and boys. The classes are separated and she addresses each group separately about puberty, upcoming body changes, etc. Sadly, it is the only information some children get because the parents are too embarrassed to talk to them OR the parents are so blind to think that they are not hearing things at such an early age.
A lot of people suggest that if you do not feel comfortable talking to your children, then go on a walk or drive so you are not looking at them in the eye while you are talking. However you do it, please talk to them so they will learn facts from you vs hearsay from school which starts WAY early. It is vital to have wide open lines of communication so they will come to you, especially as they get older and feel comfortable to ask you questions.
You say your girls are 8 and 9. Many girls in this age group are already developing breast buds, etc and they are not too young at all for you to be talking about sex. Just be specific and answer questions honestly. Don't make any part of it "bad"... this is life! They need to know there is no shame in puberty, sex, and their bodies. They need a healthy self esteem about themselves so that they are armed with correct info later on when they are in higher grade levels, etc around boys and being pressured to do things they may not be comfortable with at the time. Of course, explain STD's (yes at this age) and by 8 and 9, the know the process of sex from what they've already heard. It is up to you to make sure they know facts.
The books which have been suggested are good books. I did have some with my daughter (now 19) but I found plain communication worked best for us. You would not believe some of the topics we discuss.... some people would cringe but I am glad that she chooses me to bounce things off of at this part of her life.
Best wishes to you.