When it really comes down to it, protein powder is just a convenient form of protein to help meet your daily protein requirements. It's basically the exact same thing as whole food sources of protein like fish, beef, chicken etc.
If your daughter is struggling to eat enough food, a protein supplement will certainly help. Just use a quality protein powder that isn't loaded with fillers or artificial junk.
For a whey protein - you want to see nothing but that in the ingredients list. Whey protein, natural flavours, a natural calorie free sweetener like stevia and then maybe some salt, soy lechitin and cocoa powder. There are a lot of good products on the market. I personally use isolation from n2bm.com. A good whey isolate should be pretty much fat and carb free. Isolation has 26g protein, 1g carb, 0g fat per 30g scoop.
What I recommend is to buy a high quality blender and blend together the following:
2 scoops of a high quality whey protein isolate
oatmeal
natural almond butter or peanut butter
frozen fruit
banana
ice
chia/flax seeds
spinach/kale
Have your daughter drink that for breakfast each morning. It'll be a really substantial and easy digesting meal for her to start off her day with.