Hi A. R,
With any complaint of pain, I would address with a doctor to be on the safe side. With children it could be nothing but then it could be something. It could something simple as "growing pains" (ages 3-12) or lack of Potassium/Calcium intake. Growing pains almost always affect both legs and in no way limit the child during the day .The child does not limp during the day and there is never any joint swelling. When the child is seen by their doctor, the examination is completely normal.
There are other causes of nighttime leg pains but these usually include only one leg or the joints. Nighttime pain associated with a limp during the day or reduced physical activity should always be evaluated by the child's doctor. Pain that is made worse when rubbed by the parent, or when the discomfort is in the same exact spot each and every night should also be checked by the pediatrician.
Leg pain that occurs on a very regular basis and is associated with other symptoms, such as decreased appetite, weight loss, pale skin color, and fatigue, may be a sign of a more serious problem and should be checked as soon as possible.
Good Luck to you and many blessings.
D. K