Likely arthritis or multi-system disease such as lupus. Rheumatologic evaluation essential.
It really could be any number of things from inflammatory (arthritis) to a ligamentous injury. I would start by seeing my pcp.
Prior surgery?Swelling? Fever? Med hx? Too many posibilities. Need an evaluation with xrays.
Have it evaluated:
Have it evaluated by a professional. Depending on the severity of the injury treatment options include: bracing, casting, physical therapy, injections, and surgical repair.
Have it evaluated.:
Locking and clicking are signs of soft tissue or meniscus damage. This needs to be evaluated and treated by a specialist to possibly prevent future complications.
Is a nonspecific sign and does not indicated tears or any other pathology. However, if your pain is worsening it is important to be evaluated by a musculoskeletal physician to determine the cause and correct treatment of your knee pain.
Uncertain see peds:
You need to see your pediatrician to see if your child needs a rheumatology or orthopedic evaluation. Since she is otherwise active and has no other limitations I suspect it is physiologic and no inflammatory joint disease. Even though the pain is in the knee sometimes hip problems can radiate to the knee.