Reddit reviews The Period Book: A Girl's Guide to Growing Up (But Need to Know)

Maybe give her a book like this one to read. Try to get her out of the house(like, take walks together for chatting purposes, added bonus of helping with period pain) when she's on her period- is she uncomfortable with the period itself, or the accompanying bloating, fatigue and/or cramping? An emergency "feel better" kit of water bottle, snack bar(protein bars tend to be helpful), ibuprofen or midol and some iron supplements(chocolate optional). Letting her know that most women go through this -every month- may help. Maybe get her a little day planner and some stickers to track her periods when they happen so she can work out next potential period to feel more confident about it without being overt about her "period planner". You can get free apps that do the same thing, but it's nice to have a paper copy, and it gets you in the habit of day planning.

A friends mom did this, I was so ridiculously jealous when I found out about her "emergency stash" of feel better stuff. When I started getting regular periods my mom said "And? Want a gold star?" Feel the love.

It seems silly, but I was ten, and my mom took me out to dinner and let me ask her any questions that I had. That helped reassure me. She gave me The Care and Keeping of you, which /u/prettylittlefox already linked, and The Period Book was helpful too.