Both of my older two girls dealt with aching legs at bedtime in their younger years, and I always chalked it up to growing pains (having never heard what the doctor told KileeGiles about there being no such thing as growing pains).  I remember going through it too, as a

child, and my Mom's favourite remedy was to rub some Walkins Wonder Oil on my legs.  That always seemed to work, though it may have been as much the healing touch of my Mom as it was the Wonder Oil that soothed the ache.  One of the solutions I've found to work in my household is a warm towel wrapped around the aching leg.  I heat a towel in the microwave for about a minute (be careful not to heat too long, because and inexperienced babysitter once burned a towel :-) and then wrap it around the leg and cover with the blanket to keep the warmth in.  This almost always soothes them enough to let them fall back to sleep again.

I'm not sure about KileeGiles' doctor's advice to get an ultrasound, but with both of my girls, it seemed to end (or at least taper off) when they reached about 7 or 8 years of age.