Sounds like a Giant Cell Tumor based on the classic "soap bubble" appearnace. However, it could have been an aneuysmal bone cyst, enchondroma, osteochondroma, or osteoblastoma. The final diagnosis is made based on your daughters age, the location of the tumor and its appearance on Xray.
Sometimes MRI and/or Bone scans are done to complete the workup. In some cases, even though the tumor is "benign", a chest X-ray is needed to rule out spread to the lungs.
Treatment for the tumor itself depends on the not only on the correct identification of the tumor, but also on its size, location and your daughter's age and health. Generally I recommend evaluation by an orthopedic tumor specialist.
I had the same thing happen. It's uncommon for someone who's not done growing to have a gct... or so I was told. My tumor was in my hip and the fracture was found during surgery. If surgery is needed, it varies by doctor on the process. I 100% recommend you get more than one opinion.