My 6 yr old daughter complains of leg pain every night and also ankle pain at times... What could be the cause?

Pain in a child. - shoud not be ignored. See your pediatrician or go directly to a podiatrist for a thorough evaluation.
Let me explain. We have to be sure she have no problems by taking her to orthopedic surgeon. If he did not find anything we could label that what they call growing pain. Doctors don't know exactly what causes growing pains, but they may be linked to a lowered pain threshold or, in some cases, to psychological issues.