Hi D.,
I can only imagine the frustration and helplessness that you may be feeling when your child doesn't eat enough and sometimes loses weight. My daughter is 1 1/2 years old (has a dairy allergy) and only weighs 19 lbs. It just breaks my heart that she is below the 0th percentile, however she is super active, naughty and very healthy otherwise. My advise is don't worry too much about the numbers (I know, sometimes as a mother you can't help it, right?)
Anyway, here are my suggestions for what you can do to help with your daughter's weight gain:
1. Do meet with a nutritionist who will suggest healthy foods that are loaded with healthy fats and calories.
2. Add an extra teaspoon of olive oil to her meals
3. Whole milk
4. Avocados/ Guacamole
5. Shredded coconut and honey to her breakfast oatmeal
6. Eggs
7. Milkshakes with icecream
8. Vegetable fries (not just french fries, but sweet potato fries are great, so are veggies like bell pepers, onions, baby corn dipped in batter and fried at home)
9. Cheese
10. Nuts, especially cashew nuts and raisins are excellent
The key is not to give her more food to eat, but make sure the meals she has is loaded with calories. Extra cheese in her eggs, avocado and cheese in her sandwiches, coconut, honey and bananas in her oatmeal, mayo in her salad, chocolate syrup in her milk etc.
Finally, ask her pediatrician if there are any supplements that she may need to eat to encourage weight gain (such as omega 3 etc).
Hope this helps. I wish you all the very best.