Amy,
I do not have CF, only bronchiectasis, so take this advice with a grain of salt. My dad is a food regulation/safety reporter and editor and he has worked for government nutrition reform groups in the past. Also, I have a 6,5,and 3 year old. So those are my dubious credentials. Plus, my best friend's oldest, now seven, has always been below the 5 %ile for weight. I have watched her struggle with him as an infant. Now, she has given up, but he does not have an illness that depends on him maintaining weight--so I understand that you cannot give up.
Try to focus as much on vitamins as you do on calories i.e. less french fries, more avocado and healthy oils. You have to eat more healthy food to get the calories, but the eating habit will build and developing healthy habits when they are young is huge. My 3 yr old is my laziest eater but his threshhold for interesting foods is still way higher than most kids because I do not relent and allow day after day of pb&J and nuggets.
Bring her with you to the grocery store--a Whole Foods type store with a selection of things like gourmet nuts, olives, almond butter and coconut butter. These are all high calorie food that pack nutrients. Also, the pretty stores can get you excited about fattening delicious food--got one of mine to eat turnips and all to eat fennel.
Have her help you make the food with fun appliances like the hand held mixer: peanut butter cookies and guacamole (avocado, lime, tiny chopped onion, cilantro). Also, grating fresh fattening parmesan cheese.
I don't know if she is school age, my oldest two are K and 1st. When they come home they tend to be hungry. That is when I try the new foods: lentil stew (1/3 kids ate it), lamb with mint (2/3), papaya (3/3), salmon (3/3). I pay less attention to meal times and try to take advantage of the hungry times for the good food.
Good luck! I have to admit, doing all this takes time. For me, cooking for my kids, cleaning, being sick and some freelance editing are my only jobs. It is much harder for my working mom friends to make such a big deal about food.
Oh, one more piece of advice. Sit with her an eat, even if it is just a little. Cook the same things for the whole family if possible. I eat with my kids and make up a plate for my late-working husband.
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Melissa, 33, bronchiectasis