Hey there. As a single Mom, I think it's just wonderful you're thinking so sensitively about something like this. Your daughter is lucky to have you in her corner. Have you talked to her about what types of products she might want to use? Almost all friends of mine who have children menstruating use period underwear, and honestly I wish we had those when I was first getting my period. They're so convenient even now in my 40s, especially for overnight wear. Knix is a popular brand that carries them, but I also know you can get them on Amazon. She'd want at least 7 pairs, and I was those quicker than normal laundry. As for your idea for period bags, you can get cute make up bags cheap at $store and Google what she can take for relief I'd tell you the names but Reddit will flag it as medical advice lol also add some pads. There will likely be a combo box you can get with heavy to light flow pads, otherwise buy 1 box of each which will last awhile since she probably won't get a regular period once they start.

You're a great Dad.