In a circumstance like this (without knowing the details, which is perfectly fine), it sounds like your best recourse is to speak to the guidance counselor of the school where the kids attend, or better yet, to have your daughter report the incident of bullying. The school is required by law to follow up on such occurrences, and take legal action where warranted. They will speak with you, and protect your children in whatever capacity is warranted. This means they may have her write a written testimony rather than appearing before police, or they may have the school resource officer, with whom she would be comfortable and familiar, ask her a few questions in the school setting.
For the other incident, involving the 17 year-old girl, you should also report that to the school, rather than speaking directly to the child, as she is a minor and all sorts of things could go wrong with that, particularly if you are not a family friend or close to the family. That's tough, and I think I understand what you were doing and my instincts would totally be the same...just don't take that any further. :)
Hope this helps! I'm going to click "ready to be rated" but please feel free to ask any further questions you may have.