Yep, liners are a must. Girls start have white discharge for up to about a year before menstruation starts. The special outing is good too And maybe a heat pack for cramps.

Also pop a brown paper bag in her little purse for discreet disposal of used pads - just in case the school or wherever she is at the time doesn't have appropriate bins.

When she does get AF, marking it on the calender (maybe just a little dot in the corner of the day) just to keep an eye on when it's due etc. helps as they can be quite irregular for the first few years.

Good luck