My husband is 6'6" and 170#, he can sit and eat a 4,000 calorie meal and lose weight. Me? I can have steamed veggies and a lean piece of meat, no sauces and barely hold on to the scale without gaining more weight.
My oldest son is 9, almost 5 feet tall and 80 pounds....very athletic and skinny. He is constantly hungry. We serve him his first portion and then if he wants more, I tell him ok. I don't want food to be a control thing. He never eats more than what he needs. We HAVE to get him adult meals....well, not at fast food place, since we never do the soda, milk, etc and rarely ever fries...but at restaurants, he always orders off the adult menu.
In the past, if I think he's eaten enough, I will have him wait 20 minutes to allow his brain to catch up with the fact that there is food in his body and see if he is still hungry. He usually is, so I've stopped doing that...but I wanted him to be aware that it takes the brain and body a little bit to get everyone on the same page.
Last night we were in LA and stopped to get fast food at a local place with better ingredients than most; he ate a hot dog, fries and a 1/2 pound cheeseburger. He was satisfied, not stuffed.
Our kids only drink water - no soda, rarely ever any juice (like once every 1-2 months). We don't eat sugar. We eat food at home and he loved broccoli with Bragg's Amino Acid spray, rather than soy sauce. All in all, we eat really well, so they are getting nutritious food.
So, here are two things I will bring up:
1. You might want to get her a good multi, as many foods are not nutrient dense. Catalyn from Standard Process is a whole food supplement, as well as New Chapter's Prenatal. Not full of synthetic vitamins. If she's missing nutrients, she'll be looking everywhere to get them. Also, if she's grown that fast, I'd add a good calcium supplement that does NOT contain calcium carbonate or oyster shell.
2. I had parasites as a child. No MD could find them on blood tests. I was super skinny and was constantly hungry. My mom finally took me to a Naturopath who found I had parasites. He gave me some remedies and I was never that hungry again. Literally, I would cry because I was so hungry. I would tell my parents that I was starving and my dad would get so mad at me that I couldn't be starving because I had just eaten.
Good luck....but if she's thin and healthy, don't make food a power struggle. It will not turn out well in the end.