H.,
how about a private mother-daughter spa day or evening. Here is a link to an article:
http://www.mothering.com/saturday-night-spa
They started when the daughter was much younger and it is a regular event for them. Sounds really neat, I think, makes me wish I did not just have boys LOL
But whether or not it becomes a regular thing, it may be a wonderful special time for the two of you, with plenty of time to pamper each other, discuss all kinds of topics and have something out of the ordinary. And it can always be topped off with an ice cream treat or such :-)
Have you read The Red Tent by Anita Diamant? She touches on such a ceremony (whohc may not be exactly what you are looking for). Throughout the book various rituals and ceremonies relating to women and women's reproductive life are touched on, and it may provide you some ideas.
You say you're UU - are you familiar with the Adult RE course Cakes for the Queen of Heaven?
It's been many years since I took it, but it seems to me that somewhere in there were some references/suggestions for various rites of passage celebrations, first menstruation would be one of those. Check with your RE person, or minister, or simply search the UUA website to find more info.
Another resource that might inspire you to come up with adaptations for your own little ceremony would be either
Mother Rising - The Blessingway Journey into Motherhood by Yana Cortlung, Barb Lucke and Donna Miller Watelet
or
Blessingways - A Guide to Mother-Centered Baby Showers, Celebrating Pregnancy and Motherhood by Shari Maser.
While it may appear that I am jumping the gun here, as she is just about to menstruate but is far from becoming pregnant and a mother herself, these books celebrate the women's bodies, the natural way of things, they contain many little ideas for honoring and respecting our bodies, to be comfortable with our bodies, along with recipes, songs, etc.
I think you might well be able to look through these books and see that there are ideas in there that you can easily adapt for your specific purpose.
Have fun with it, nice to see a fellow UU on this list :-)