By definition, will some children always be stunted?
You are correct, that under that definition; unless there were no variation (which would be impossible), there would always be individuals that fall below 2 standard deviations. However, "stunted" under this definition would not be a problem if variation was extremely low. So yes, it would be unfixable by definition, but if variation fell, then the "problem" would be fixed, because it would no longer be a problem.