My daughter is 12, her knees hurt and now pop when she walks. What should I do to help ease the pain in her knees?

Ice, Tylenol (acetaminophen) When one has knee pain, it is ideal if the source or cause of the pain can be determined. This is best done by her pediatrician, family physician or orthopedist. In the meantime, the pain can be addressed with r.I.C.E.: rest, ice, compression and elevation. Additionally, tylenol (acetaminophen) may also be helpful but this is only a symptom reducer, not a cure. See you doctor for the cure.
Homeopathy. I agree with the first answer. Being evaluated is always the best first step. If she is sports oriented she may be having some overuse issues. Homeopathic remedies arnica, rhus toxicodendron and bryonia can do a lot for orthopedic discomfort. 30c or 200c- 2-3 pellets 2-3 x a day for a few days may help with the pain.