I also learned that true anorexia, as opposed to just over doing a strict diet, is a hereditary brain disorder, and if not genetically predisposed, it's not possible to develop anorexia beyond eating too little and saying whoops, I think I went too far. Not that getting to the point of whoops is insignificant, just that it's no more a battle than figuring out how little is too little, and how much is just right.. People with the real deal anorexia, from genetic factors, will not say whoops I went to far, and they'll more likely attribute their fatigue, fainting, and heart palpitations to something they're doing wrong with their food manipulations, like eating the wrong thing, or too much of another thing, anything but eating too little.
You can probably relax if you're concerned that your child has been exposed to a "thin is in" culture, and is about to go off the deep end to full blown clinical anorexia. It's hereditary, so if it's not in the family's history, it's extremely unlikely the child could get it if they tried. Sure, educate yourselves (don't take my word for it) and keep watch over your children and help them understand what a healthy diet is, and what it can do for the common person, as well as the athlete.