Ok, I know you're going to think I'm crazy, but I believe I got this information from the Dallas Morning News in the "Ask a medical question" section (not sure of the actual name of the section but I believe it's on the same page as the Dear Abby/Ann Landers columns).
If your child is actually having leg cramps, not just growing pains, this remedy might help. Now, this is one of those old wives tales home remedies that has been passed down for decades, but it has even baffled scientists because it truly does tend to work. Take a bar of soap and place it between your child's mattress and her fitted sheet near the foot of her bed. That's it. If her problem is cramping, it's possible she won't have any more problems with leg cramps after that.
Now, growing pains, I've heard, are different because that's actually caused by the stretching of skin and muscle, but I would guess trying this might be an option (???).
I know it sounds crazy, but my husband was suffering with leg cramps nightly for a while several years ago and my mom told me she read about this remedy and gave me the article from the paper. Neither of us have had leg cramps since we've put the bar of soap in our bed. It's just a new unwrapped bar of ivory. My mother-in-law had a circulation problem she ended up having to get surgery for and she had terrible cramps. She was even having them in the hospital after she had her surgery. We put one of those small hotel size bars of soap under her fitted sheet on her hospital bed and the cramps stopped.
Again, I don't know why or how it works, but for everyone I know who's tried it, it has worked. If you give it a try, maybe it will work for your little one too.
Good luck! :-)