8 sounds so young but I do know that is the trend....girls entering puberty younger. You might want to check with her doctor/pediatrician but I had a couple other thoughts. First, I remember being quite frightened when I started developing because I thought I might have cancer and was too scared to bring it up with my mom. One of my friends finally put my mind at ease that it was normal.
Second, she probably knows more than you think but also probably has some misinformation too. I was a high school health teacher in my former life so my students were older but I always was interested how much diversity in knowledge they had. Always some misinformation too.
Having no idea how comfortable you are talking with her or what you have already talked about, maybe bring it up if you see a commercial on for tampons or pads. Ask her if she knows what they are for and if she knows what a period is. My advice is keep it really simple so that she isn't scared but tell her as much as she asks. Then maybe just tell her that you heard more girls are getting their periods and developing breasts younger and younger and ask if she is sore or is intersted in a training bra. I'd probably leave sex out for now unless she asks. Books are good too if you aren't as comfortable talking to her. Good luck!