I'm so sorry your LO is still feeling so sick. I don't think it is uncommon for kids to lose weight when they are sick, though it definitely is concerning. Is she still refusing the breast? I'd think that would be the best thing for her right now--rich in fat and nutrients as well as antibodies to help her get better. If she won't nurse, would she drink expressed breastmilk from a cup? As for the whey protein powder, I'd think you could add that to pretty much anything in which it would dissolve. Another good way to add some fat/extra calories is with olive oil or canola oil (these kinds both have lots of the healthiest kinds of fat). I used to put a little in DS's oatmeal. There is a powder called Duocal that will dissolve in food without changing the taste or consistency for added calories. It is made for people who are unable to keep on weight due to illness (like cancer patients and so forth). It has no protein, only fats and carbs. Your doctor I believe can prescribe it, although you can also order it online (it's a bit pricey). I hope your LO is feeling better soon!