I think it can be reversed fairly quickly in a generation or two with proper nutrition so nothing permanent. Our ancestors seemed to maintain those taller, more robust features for centuries then it took just a few generations in the modern era for the current state of affairs to pop up. Not much different from what Weston A. Price noted with a lot of indigenous cultures once they adopted modern diets:
Often took a single generation for that kind of degeneration to occur. But I'm quite confident from what I've seen anecdotally that the reverse will be similar much like
this experiment with cats. Just feed your kids well, instill an awareness of health and healthy eating in them that they will in turn pass-on to their kids and lobby for a future, developing Somalia to have good nutritional guidelines and laws like going so far as to ban grain-fed meat or soy and corn fed chickens and sure as heck not advising people to follow "The Food Pyramid".