The things which I believe help our son most are:
GSH (Glutathione) which reduces inflammation, increases appetite for many, and provides powerful antioxident protection.
Probiotics (daily) like Reb mentioned, just seconding that and how much it helps the gut to absorb nutrients.
DHA in the diet and as a supplement (ground flax can be snuck into a lot of things). We like Nordic Naturals strawberry liquid gels.
Tumeric has helped our son with stool consistency quite a bit. We like New Chapter Tumeric Force one cap daily.
Anti-inflammatory diet can really help. Getting white sugar, corn syrup, white flour and hydrogenated oils out of the diet (which means the supplemental drinks they recommend) can help weight gain believe it or not. Many of the adults on this board have experimented with this and had great results both in terms of weight and lung health. Check this discussion out for some great info. <a target=_blank class=ftalternatingbarlinklarge href="http://forums.cysticfibrosis.com/messageview.cfm?catid=5&threadid=18569&enterthread=y
">http://forums.cysticfibrosis.c...d=18569&enterthread=y
</a>
We go through the toddler issues too on and off. It is so hard to know if it is just normal kids development stuff or something to freak out about. I agree with finding the healthy favorites and doing them as much as possible. I sometimes do a drawer or basket in the cupboard with healthy choices like Clif Bars (all natural no cornsyrup and great calories), trail mixes, chips for dipping, cheese, ect. Then he can pick some times. Toddlers love choice. There are tons of peanut butter ideas coming to my mind from making sandwiches and cutting them up into special shapes to adding flax seeds to PB and tossing in some rice krispies and honey then rolling into balls and dusting in cocoa powder (can you say run on sentence). Cooking or preparing snacks with her may really help interest her in food. Coconut oil is a great thing to add to oatmeal and rice...cooking breaded chicken in it is really yummy (www.tropicaltraditions.com). Turkey rolled with cream cheese is a good boost of protein if she is eating on the run.
I also noticed with Ben if we let him drink too much between meals he won't eat as much. I would only offer drinks at or after meals/snacks.
Best of luck to you...