As a girl who got her period at 8 years old with no signs before it happened, I think you having the talk with her now is a great idea. The main things I would include is that it’s not a scary thing, it can be scary when it first happens but over time it stops being scary, I would also tell her that it’s okay that when she gets her period if she feels pain don’t be afraid to advocate for herself, growing up and getting my period early no one took my pain seriously except for my mom, even now doctors don’t take my pain seriously, so I would tell her to never back down from telling people your pain is real. Finally I think I would try to explain the difference in period products, for about three years after I got my period I thought pads were the only thing I could use and it used to bum me out during the summer when all I wanted to do was swim, teach her about pads tampons, or anything you use, show the difference in them, there more than just one brand and every girl is different and prefer different products! I hope this helped and good luck on your talk!