BH - What you are experiencing is an increasingly common concern in the world today (as has been pointed out in a couple of different ways). First and foremost, I would advise you ask yourself *why* you are concerned.
Non-Christian religions are dismissed out of hand and stigmatized by the predominantly Christian society in which we live, shunned as being everything from misguided, eccentric humanism to outright service of Satan. However, sensible people such as yourself can easily look at the evidence at hand and decide whether or not a reasonable course of study and practice is being undertaken by your child.
First...if your child is claiming to go around casting spells on others to get something that *she* wants - she's not learning properly. If she appears to be masking other sociological shortcomings by becoming a witch, she's not learning properly. If she drops all her other friends, withdraws, starts drawing pictures of Beelzebub on her bedroom floor...*then* you need to step in and get some counseling.
On the other hand, if your daughter keeps her grades up, continues to be socially adept and doesn't start becoming outright delusional, then consider that she might be growing up and beginning to make decisions on her own that truly reflect her increasing sense of empowerment in her own life and the world as she sees it. For young girls especially, Wicca is an attractive path, and rightly so. It is a religion of female empowerment and responsibility, a religion which reveres both goddess and god. It is a path of reflection and discipline, of learning and, yes...faith.
Wicca is indeed a valid religious path. Like any other religious path, it involves belief and visualization of things that others might not see (Christian example - big serious invisible man in the sky...and son). Wicca, however, involves some more concrete things at its core. It involves a reverence for the Earth and all of Creation. It involves knowledge of the bounty of Nature, her plants and stones and energies. It involves respect for your fellow beings and other living creatures. It involves tolerance and acceptance of different races, gender preferences and religous paths. You'll never have a Wiccan knocking on your door at 8:30 on a Saturday morning trying to sell you magazines or insisting that you are going to writhe in eternal agony if you don't believe what they believe.
The most critical thing, IMHO? Find your daughter a good teacher. I can probably help you with this to some extent as I have a couple of contacts in your home state, and could possibly drum up a good class or two for her. Use the Web, but use it with a grain of salt...there is a great deal of incredibly stupid crap out there. A good starting point is
www.witchvox.com. The operators of this, the largest and most popular of Pagan web sites, are vey grounded, educated and extremely pleasant people who dedicate themselves to the dissemination of sane and accurate information.
The same codicil about quality goes for books. There are both good ones and drivel. Our coven degree system maintains a good reference list of required reading (for our own members, that is - we make no claims on the contents being the be-all or end-all), and I'd be happy to send it to you.
My wife and I have run a coven for many years now, and my wife is only the third teacher of the fundamental Wicca classes here in KC which go back some 20 years now (and both of the other teachers are our very good friends). Most of our discretionary time is concerned with spirituality through Wicca, serving our local alternative religious community, and providing open public access to rituals and community socialization on the eight major sabbats of the Pagan year. All this to say that I have more than a little experience with all of this, as does Wolf.
As with any undertaking your children may become involved in, parental involvement and attention is essential...even if you don't approve or agree. Only *you* will know if your child is safe and sane. If you need counseling or advice, or simply to talk about what's going on so you can be more well informed, there are a few people here on The Cellar who will be more than happy to serve as a resource for you.