As long as she takes it out after the right amount of time there really isn't anything wrong with it. I feel bad she's already started her period!!! Poor thing! I wouldn't let her sleep with one in either. Maybe only use it when swimming or when she's playing sports. They do make the super skinny ones that I remember using when I was younger at first.
I agree with the above poster. I'd try to limit it to her swimming and active sports. She's still changing a lot and any kind of infection or bacteria will not be good. I also feel for her starting so young!! Are you just wanting to teach her or does she want to be wearing one? I knew as soon as I was a little older I started using them, around 14ish I think.
No contraindications, given that her periods have begun, obviously. Just be sure she knows to change them when she goes to the bathroom, and has a variety of sizes to use so she can begin to judge her flow so she is not changing ones that are totally dry or flooding ones that are too small. Have her use pads as backups for a while when she is figuring this all out. I wouldn't limit her to sports only, but I would suggest she wear pads to bed for at least a while, and let her decide how often to wear them in the daytime vs. pads.