As a neopagan and former practicing wiccan, I think you'd do better to specifically look for information about cults and research those and then make decisions on how to go handle your story from there. Wiccans have nothing to do with sacrifice of any kind, and a book that refers to wiccans or alleges that they are involved in destructive cults would be both inaccurate and harmful to a religious group that gets targeted plenty. I don't wear a pentagram in public and most of my family is unaware of my religious affiliation because I don't think it's worth the ugliness that will come from it.
Wicca is nature-based, and traditional Wicca has very formal rules and traditions. No one under 18 would be allowed to join. These groups have very strict rules of operation and conduct, and would require extensive research to do well. Neopagans who call themselves Wicca but don't follow traditional Wicca rules tend to be nature-based, celebrate the new and full moons, and mark the eight major turning points of the season. They may meet in groups or practice alone. They may use candle magic or other rituals and ceremonies. The notion of harming another person is absolutely wrong. Acting in a way that imposes your will upon someone else is also seen as ethically wrong. The term Neopagan is more general and the ethics and rules become looser. Some may base their practices on old Greek gods, Egyptian ones, or really, any god that ever is or was. There are Christian Neopagans. Not sure how they make it work, but it works for them.
Please be careful in how you write your story. If you are looking for a very, very general information site that covers what different non-mainstream religions cover, I recommend http://www.religioustolerance.org/. It's not detailed, but it covers the generalities in a neutral, open manner and is built for people who might know nothing about a specific religion.
It's worth noting that most religions have some pretty crazy offshoots. I'd suggest starting with the notion of cults on Wiki and seeing if you can find one that sounds interesting to you and going from there. I'd actually suggest focusing much less on Satanism as a key word and looking more into extreme forms of Christianity ... mostly because you'll find more hits. It is the dominant religion in the US, so there's a larger number of people who happen to identify as Christian who are also in/run highly destructive cults. And a cult is a cult. Regardless of religious denomination, you'll see similar patterns in controlling and examples of what a charismatic but insane person can do to a group.
Think about what you want out of this religious cult. Do you want organization and cohesiveness? Are the parents part of a group or are they acting alone based on their own personal beliefs and/or religion. Why would the parents choose this particular religion? Are they looking for self gain or are they looking to control their child? This will help you decide what sort of group you 'need', which should help you find a real life example that mirrors what you're looking for.
If your parents homeschool, exercise a lot of control, cut off outside contact and practice extreme religious views, you might even have better luck looking articles on some of the nasty things that can happen when parents homeschool (They're a serious, serious minority in the homeschooling population but ... again ... any group big enough and you start to see bad eggs).
If you go the homeschooling route and look towards religious fundies as an example of how religion can become twisted and unhealthy, this article might get you started: http://www.alternet....christian-homes I won't vouch for its accuracy. Alternet has some pretty hokey stuff sometimes. But they also list support groups and have some personal anecdotes. One of the people interviewed left her form of religious fundamentalism and went over to Catholicism. Which (based on your post in Guppy) sounds like it might be a little like the recovery you want your character to go through.