Reddit mentions of Period.: A Girl's Guide

>I had this whole thing planned. I was going to get her the book, "Our Bodies, Ourselves", buy her some feminine products, some chocolate, and just be as open and supportive as possible

You can still do these things! Maybe choose a book more suited to reading level, but they are out there.

I was trying to find my favorite kid book on menstruation and I THINK this is the one -- but maybe check it out at a library first. As I remember it was really easy for me to "get" without feeling gross and shamed.

You sound like a fantastic mom. I think as long as you are a parent that she can come to (maybe with encouragement) when she's hurt, confused, or cranky, you should be great.

Maybe approaching this as perfectly natural would bring you closer together? Since she is quite young, maybe start with just a general book about bodies developing? You can work your way up to the more complicated stuff. Don't freak out about this, as that might make her believe it's unnatural and wrong -- as a young child, she's probably only going to have a really basic understanding of what's going on. Answer her questions, but don't make her afraid of what might be lurking in her chest/privates. If she's not ready to talk about any of this stuff, tell her that's ok, but make sure she has access to books and that whenever she wants she can change her mind.

Don't put the cart before the horse, but once the horse is ready, don't spook the horse with a scary fucking cart full of misery and periods!

Also -- if your sister had to point them out, are you sure it's not possibly a little lingering baby fat? It may have been early and I didn't notice, but I think I was like I was maybe 10 when I started "developing breasts" -- but it's not like they ballooned over night. I want to say it wasn't noticeable until high school. I was ashamed of them, of course, but my mom let me wear what I wanted -- if she favors baggy clothes or tighter shirts, (or better yet -- feel fabulous in any outfit!) all you need to do is support her and frequently remind her that you two can talk about anything.

If someone bullies her, makes her feel like a "freak", let her know that you can handle the problem, in a way that will not draw attention to the situation, especially if she's a shy girl. Don't demand she wear a bra and be an expert on menses. You buy her clothes, so if she's not ready for the undershirts, maybe try to find materials that are a little less clingy, or buy larger sizes because she's growing into such a lady! The idea of a regimented, rule-based approach to this might make her feel overwhelmed. Just give her the information and resources she needs :)

Curious -- have you noticed any other pre pubescent signs? (Hair under there, body odor, etc?)

TL;DR: You'll be fine. Don't give her any information she isn't ready for or doesn't ask for. Don't talk about it in terms of HAVING to do stuff -- wear bras and what to do when it happens.

online resources:

http://www.beinggirl.com/my-period/first-period/

http://kidshealth.org/kid/grow/girlstuff/period_school.html

http://www.amazon.co.uk/Girls-About-Periods-Growing-Up-Stuff/dp/0340878282

http://www.amazon.com/Period-Girls-Guide-JoAnn-Loulan/dp/0916773965

http://www.celebrategirls.com/books.htm

also, expect her laundry to be kinda messy. its hard to figure everything out at first, and it may take awhile (years, sometimes) to really get a handle on how messy it can be. sheets are going to have stains on them, her underwear, pants, etc. the longer the stain sits, the harder it is to get out. I suggest getting a few black pairs of underwear for her that she can wear during that time. stains wont show up. the laundry thing can be very embarrassing. If she doesnt already do her own laundry i suggest that you teach her.

try posting this to female centric sub-reddits, such as, r/women, or r/twoxchromosomes.