My youngest SW CAHer was underweight as an infant/toddler.  He is now at 50%  for weight and is a really great eater,  his favorite dinner is salmon and spinach salad.  He was the kid that would not eat anything, his weight was also off the chart low...I did give him the instant breakfast with his moring milk, but I am not sure if that helped or hurt as he loved his milk and sippy cup.  I would say as long as they are growing and healthy try not and stress about the weight,  I know it is easy for me to say now, as my guy is now 8 years old.  I did worry but I think that you can’t let your child know or make meal time stressful.  Just keeping offering healthy food and let them do the rest.

Hang in there!