The rule of three
You're stocking three places: her school bag, her bedroom drawer, and the bathroom cupboard. Buy for all three at once — running out at 7am on a Tuesday is the worst version of this.
Keep receipts and start small. Bodies and preferences change; you don't need a year's supply of one brand on day one.
Bathroom cupboard (home base)
You don't need tampons, cups or discs yet — those come later when she asks.