Thanks everyone... for some reason whenever I've thought of salt depletion I've always thought of symptoms associated with heatstroke, symptoms like muscle cramps, exhaustion, stuff like that. Never did contributing to growth problems even occur to me.
She is a very medically complex child and has SO MANY SYMPTOMS that can point to any number of problems: her seizures have been associated with nausea, vomiting, and afterwards she has headaches and is very tired. She has migraines, which also can cause extreme exhaustion and nausea. One of her seizure medications can reduce her tolerance of the heat. Another medication is notorious for affecting growth and appetite but she cannot function at school without it (believe me, we have tried. Over and over again. And the enduring fact remains anyway: the kid eats like a horse!) The fact that these symptoms can all also be cf-related absolutely drives me crazy because I cannot discern the difference so much of the time!
Her developmental disabilities limit her ability to communicate how she is feeling about any of this stuff, as well as her ability to comprehend how important it is to fully cooperate in certain aspects of her care that embarrass her (i.e. showing us the poop. She finally achieved potty autonomy at age 5 and is now known to sometimes take it very personally when we need to know any bathroom details.) So it is very hard to figure it all out, very frustrating.
So, supplementing the salt certainly seems worth a try. I think I will try the product Missa recommends, that sounds very good... maybe a very tiny amount to begin with (in hopes she won't notice) in plain water and work up to the dose recommended. This seems like a great place to start given her preference for plain water. Maybe the tablets if that is not enough. She does not often ask for salty snacks, but there are several she does like... key will be moderating how much her brothers and father eat (dad has high blood pressure!)
I do wonder if her increased problems with stomachaches over the last few months has come from my deliberate efforts to get the salty junk food out of the house. I never really thought about it having an effect on *her* ...I bet she was eating more of it than I thought, and even stuff like store-bought cookies often has a very high sodium content, which is why I had to stop buying most of it (when it's in the house, dh eats it.) The treats I bake, for example, don't. I just did not THINK of this. I make her separate fatty stuff and didn't think of replacing the salt.