Coach, it sounds like you have the right ideas with food and information provided. Keep stressing "fueling the machine," if worried about consuming calories, maybe use the analogies of high quality fuel and empty tank? I also like the idea by the poster who suggested strength training.
I flirted with anorexia for some time and quite honestly - I loved it when people would tell me I needed to eat. Like all my (sick) efforts had paid off. If a coach had told me I weighed too little, I would have seen this as a great victory. I was also of the mentality that if 120 was good, 115 was better, and 110 would be better yet - but I never enjoyed hitting those weights, just saw it as "OK, now I have to get to __ pounds." I gradually figured out I ran better with a little fuel in the tank. Maybe emphasize this to your girls?