J., my daughter was also very low on the weight chart, at least relative to her height (at one point 25th percentile weight, while her height was over 100th percentile).
Like you, I gave her high calorie foods, but she had very picky phases, and also "got full" quickly. Basically, we built up her meal-time appetite by providing snacks constantly (which is technically a healthier, and more natural way to eat). I'd put out peanut butter with apple slices for dipping, chunks of cheese, little sandwiches (cut into small diagonals so that they wouldn't seem like a "meal", mini-bagels with cream cheese or pizza toppings, etc. I'd also put out whatever we were having for dinner (cut up into small pieces with toothpicks in each piece, then I'd set it on the coffee table for some time before and after the "main" meal).
It was a little weird -- sort of a non-stop cocktail party, but she did end up consuming a lot more food, and eventually moved into the 50th percentile (still very thin for her height, but a much healthier weight). Also found that you can make the quantity of food at dinner appear less intimidating by serving the food in courses, and in smaller portions, rather that putting everything on the plate at once.
I realize that some of this might be tricky as you have other children around and might not wish for them to snack all day. Mine is an only, so luckily didn't face that particular issue. I don't know if this helps at all, but it did work at our house. Rather than "ruining her dinner", these snacks seemed to build up her capacity for food. At ten years old she will now eat about six tacos in one sitting, or an entire steak plus veggies plus potatoes plus desert. And again, she's still super-thin, but healthy-thin. Best luck!!
P.S. My daughter also refused (and still refuses) to drink any "sweet" drinks -- including juice, PediaSure, soda, Gatorade, etc. She will drink milkshakes though -- have you tried putting the PediaSure into a milkshake? I swear sometimes I think I'm living in a parallel universe as I've spent so much time "sneaking" fattening foods into my kid's diet. People seriously look at me askance at the grocery store when I tell my daughter that we can't buy the LowFat ice cream, or the "fake" butter...