I haven’t reached this point yet but my top tips are:
1. Have a starter pack of pads at home for her now – don’t wait until they are needed. You can buy packs on Amazon or in any supermarket etc. There are some specifically aimed at young girls.
2. Get her something nice to carry spare pads and a clean pair of pants in her school bag. Maybe put in a wee nappy bag so she can put a used pad in it if there are no bins (some school toilets don’t have disposal facilities in the loo).
3. Make her comfortable to talk openly about it with you. It’s just biology after all. (Easier said than done though.) Maybe take her shopping to choose pads etc.
4. Love the water bottle idea.
5. There are apps she can use to track her period. Periods can be pretty erratic at first and should settle dow after the first year.
I hope this helps.