You know, this is a slippery slope that clergy of all religions have walked for centuries. There is certainly a need for confidentiality with any religious leader. After all, if we, as clergy, are to offer effective counsel to those who ask for it, those folks have to know that we will not betray their confidence.
On the other hand, there's the matter of doing what's right. If someone is being harmed, and you know about it, most Pagan traditions would argue that you have a responsibility to speak up and put a stop to it.
When someone comes to me in confidence and asks to speak to me as a priestess, it's typically because they need advice. They're trying to decide what to do about a potential job change, they want to know if they should go back to school, they need help communicating with their spouse or partner more effectively, and so on. It's rare that anyone has come to me with anything as dramatic as what you're describing, but here's what I would probably do in your circumstances.
First, I'd ask myself why the person chose to confide in me. Do they just want someone to talk to? Are they hoping I'll offer advice? If someone is in danger - either the person I'm speaking with, or someone they know - and they've come to me with their concerns, it's because they want help.
Next, I'd figure out the seriousness of the issue, and how it impacts the individual or the community at large. If someone comes to me and tells me they've just been diagnosed with a fatal disease, or that they're in recovery for addiction, that's no one's business but their own. There is no reason to share that with anyone - and chances are that they just want someone to listen to them. I'm happy to do that.
On the other hand, if someone comes to me and tells me they know their neighbor is abusing a child, or that their brother has killed someone and hidden the crime, then we're talking about not only an ethical and moral obligation here, but a legal one as well. While you can't be forced to speak out, most traditions would argue that it's your responsibility to do so.
Also, depending on what state you live in, there may be laws that require you to report certain situations, regardless of your status as an ordained clergy person. For example, in some states, anyone who has knowledge of the abuse of a child is required to report it to law enforcement, no matter who they are.
Finally, in any of the cases above, I would counsel the individual to seek help with law enforcement or other appropriate agencies. Perhaps you could sit with them and offer support while they make that phone call, or while they speak with social workers, and so forth. I've driven a battered woman and her children to a shelter, because she was afraid to do it herself, and couldn't take that step on her own. Empowering someone else to ask for help is often what they need you to do.
There's a great article from back in 1985 by Rev. Dr. Marie M. Fortune, in which she described the dilemma of a pastor who was made aware that a teenage girl in his congregation was being sexually abused by her father. Although the girl was terrified, the pastor told her how glad he was that she had come to him with this information. He also told her that although she didn't want anyone to know what was happening to her, the only way to stop her father from hurting her was to get other people involved. After much discussion, she finally understood that if she wanted to be safe, other people had to help too - and she called Children's Services from the pastor's office. The father was charged and convicted, and later went into treatment. The girl herself was able to get counseling from a qualified mental health professional. The pastor was able to help the girl to help herself, and so did not violate the trust she had placed in him.
Certainly, I would say if you believe a crime is being or has been committed, you've got a responsibility to the victim to make sure they get help. Because not every case is the same, you'll have to weigh the balance of your obligation for confidentiality against your role as a leader and helper of the community. It's all part of being clergy, and it isn't always easy. However, if you follow your ethical guidelines, and those of your particular tradition, you'll be far better equipped to make the right decision.