I also agree wholeheartedly that your thin son does not "need" the snacks and junk food like the doctor suggested. There are healthier alternatives that your son can eat to try to keep his weight up, that are not adding trans fats, cholesterol, and sugar to his diet.

Thin people have heart attacks, high blood pressure, high cholesterol, and cancers that could have been avioded by regular exposure throughout their lives to the natural cancer fighting properties in fruits and vegetables.
For one, have you thought of possibly getting some nutrition shakes like Boost or Ensure for him to drink once a day? Basically, just giving him a little extra nutrients and protein in a quick drink form. It would be a quick, painless way to get in about 200 extra calories each day.
As far as getting kids to eat healthy, I really do think that part of it is in the "presentation" of the healthy snacks. A bag of celery down in the crisper doesn't really look so good-but if you have them already washed and cut into sticks, and served with a spoonful of peanut butter-it looks a little better.
The same goes with fruit. I often cut up an apple into pretty slices on a plate, and give it to my kids with a spoonful of peanut butter, or fruit dip, as an after school snack. It is much more kid friendly than the whole apple hidden in the fridge bottom.
Also-I understand that small children need softer foods-but there are healthier alternatives to Newtons. My son when he was 1-2 years old loved small slices of banana cut up. In the health food section, there are also the natural and organic snacks meant for preschoolers-cereal bars, crackers, etc.
Another important thing, besides the food, is to encourage family activity for everyone. My husband and I take short walks around the neighborhood with the children when it is nice. Maybe your daughter would go on a walk with you a couple of times a week?
At this point in the children's lives-it is best to promote "health" in general, rather than stressing that one child needs this and this to gain weight, and the other child should not have this and this because they need to "lose" weight.
