Oh boy… We’ve struggled with this.
Your dd seems to have a low-fat diet. I like the trail mix idea that PP had posted. Let her put in stuff she likes, and try to include high calorie items.
Add fats when you’re making her meals.
Try exercise. When dd was little and quite underweight and a picky eater, we would take her for long walks around the neighborhood and then she would come home and eat at three egg omelette! She gained weight that way… I think the exercise stimulated her appetite.
Could you market a smoothie as a milkshake?
It’s hard to know what to do in cases like this… DS is a naturally lean person and favors fresh fruits and vegetables and doesn’t like a lot of fat in his diet. He doesn’t eat dips, either. I feel like that’s better for him in the long run, though I worry that he doesn’t eat until he is famished. Then he will eat a lot… I tend to push food on him because we went through a big scare last year with sudden weight loss and a gazillion tests- turned out to be nothing and he gained weight after that, but it made me scared to let him do his thing.