For whatever reason, I've always been extremely open and found it easy to discuss sex with just about anyone. My mother was fairly open with me, and I appreciate that, because she was pretty prudish herself! I know she didn't want me to be in the dark like her mother kept her though. I'm honestly not quite sure why it's never been embarrassing for me.
Anyway, I felt like I had really jumped a hurdle and could talk about sex with anyone during my first year teaching. I was given a "provisional" certificate in special education, because they were desperate and they needed me to teach 3 regular ed English courses and serve as a the special ed teacher or class liason in 3 other classes. One of those was Health. The lead teacher was a young male coach. I was also young -24 to be exact -and the class was made up of 9th and 10th graders -all but 3 were boys. We got to the section regarding reproduction and the coach acted like he was about 10 years old. I was horrified! A boy in the class WHO WAS ALREADY A FATHER in 10th grade asked me, "What is a period? I don't understand how periods in girls work." The coach then put his head on his desk! I just looked at him and sighed. I never thought I would be drawing the typical picture of a uterus and ovaries on a chalkboard in front of a group of high school boys, but you just never know what awaits you! I proceeded to explain exactly how a menstrual cycle works, why there is a period/blood, how the egg gets fertilized and when, etc. Sadly most of these kids had never heard any of this before. The girls just know they had started and it meant they could get pregnant and every month they had to use tampons. Whatever residual embarrassment I had ever had around males regarding menstruation disappeared that day.
So -for you it may never be an entirely comfortable experience, but at least you want to do it! My mother always had good, scientifically-based books and pamphlets on hand. You have to be VERY careful with this, but depending on if you attend a church or are a member of any type of religion, some DO offer good sex ed courses for pre-teens. I'm a Unitarian Universalist, and we start in kindergarten through high school with age appropriate sex ed called OWL (our whole lives). You may want to do something as simple as getting a medical anatomy textbook and leave it lying around. Believe me -your kids will look and ask. Don't ever think you're starting too young though -by 2nd grade your children need a basic understand of how babies are made and born. Basically you just have to sit down with them and dive in! Don't worry if you're nervous or blushing and embarrassed! Far better to be that way and do it than not to at all!