Same issue here! My daughter actually lost 5lbs when we first start medicating 3 years ago (she is almost 9) and she has yet to gain anything beyond 2 lbs a year, but she isnt below the line so at least we are maintaining.
She loves PB&J and cheese quesadiallas. (I know I spelled that wrong!) She also loves yogurt and snacky crackers. I make quite a few of PBJs and ques every few days and put them in the fridge. You would be amazed at how much they will eat when what they like is ready and they can grab it themselves.
Another thing I found is that by about 9pm when her meds wear off she is really hungry and will eat and eat. We have thrown bedtime out the window for food. If she is hungry and is eating, she is allowed to stay up and snack. She makes up for a lot of calories she missed during lunch and dinner. Yes, she may not go to bed till 9:30pm, but she is getting the nutrition she needs.
For dinner, if she doesnt like what we are eating after trying at least a few bites, she is able to get something of her choice. I feel when working with a child who needs to gain weight, some things are less important, like teaching your child to eat what is on the table.
Her doctors have said that although this isnt the perfect plan, what she does like to eat does have nutrients and let her eat it if that is what she wants.
Good luck with whatever you decide to do. It definitely is a daily battle and you cant keep it to the back of your mind.