Bananas at bedtime!!!
or Gatorade / other potassium supplement
((Bananas are one of the most absorbable forms of potassium... but if your daughter is allergic to bananas/ doesn't like them... any kind of K+ supplement should help. Ditto, fortified cereal or bananas in the morning will help during the day).
So how muscle contraction and release gets kicked off is:
Ca++ (calcium) whenever a muscle needs to contract
K+ (potassium) whenever a muscle needs to relax
(actually, rigor mortis is caused by the abundance of calcium from the bones leaching into the bloodstream/muscles and causing them to contract, but with no K+ left in the bloodstream after a couple of hours, there's nothing to kick off the chain reaction that makes muscles relax. Only after all the calcium in the blood has been used up does rigor mortis fade. It's a pretty cool.)
Growing pains are caused by a variety of reasons, but it's usually that the bone growth is ripping the muscle (just like when we exercise, not bad ripping, but the kind that leaves you tied up in knots of ouchiness or curled up in a ball screaming from the cramps depending on how off your electrolytes are). The muscles then cramp to the bone, because there isn't enough floating potassium in the system to relax them and lactic acid is built up from the torn muscle cells.
ANYHOW.... Increase her potassium consumption, ibuprofen to reduce inflammation, and massage (upward ONLY, you don't want to break the gates on the veins by massaging downward on legs), and lots of stretching. AKA... treat her like a runner.