I started my periods at 9. My daughter was 10.
Period pants are a good intro because they look like underwear although you might have to try a few brands to find the ones that fit best for her. My daughter has Primark ones she quite likes but we had to go up a size in them. I have some from Flowette for under my swimming costume the fit is weird and the material varies between the three styles.
Reusable pads are a good thing to have a few of, because they are softer against the skin and no perfume. The patterns can be quite appealing because some of the people selling them have gone out of their way to choose unusual fun fabrics. Most sellers do a mixed starter pack and junior options.
Honestly my daughter prefers incontinence pads to deal with hers because they are easier for her to get into her pants, she has additional needs and her flow is heavy she hates those super thin ones.
They sell little square bags on temu and probably amazon that can hold two pads (probably more if you are using the thin ones and a pair of underwear). These have been in my daughters bag for years now. But that bag is inside a make up bag for her because she also needs nappy bags to dispose of the used ones, wipes to clean up the inside of her thighs and roll on deodorant because she soaks her clothes in sweat during that week, she only remembers to reapply the deodorant if she sees it when she changes the pad.
Worst case scenario she can tip the wipes and pads into her bag and put the dirty knickers into a nappy bag before zipping them in there. She would hate the idea of them touching her bag.
She has gone through her clothes enough times that she has leggings and spare socks in her bag as well.
We keep supplies of chocolate because she cant control her blood sugar very well during that week and she is pretty volatile the rest of the time.
Keep her supplies in something she has picked, you know your child she might appreciate something pretty in the bathroom that is hers, she equally might be mortified at drawing attention to it and prefer some that completely blends in with the decor. Also let her help build the contents she might decide to add things you wouldnt have thought of. If she is very young you might find liquid paracetamol or ibuprofen useful. Mine just swallows tablets now.
If you have boys in the house, including dads normalise talking about periods with them too to reduce the feelings of shame around it. My middle one was so used to periods because of his sister he kept them in his bag in case she needed them and turned into some weird unofficial pad dispenser for his friends because they all knew he would be able to produce one when required.
They sell cute hot water bottles with animals on them with a waist strap that might be good to have in the house if required. My daughter keeps those cooling gel straps for migraines in her bag to use on her lower back, although not sure if she likes it for pain relief or for temperature though.
Usborne books had a good guide for periods but I cant remember the name right now.
You also might want to reach out to her school and find out what their procedures are, like do they have a red box. You may find they can then mention them to the group as a whole then rather than relying on kids being brave enough to ask for what they need.