At my eldest daughter's last hospital appointment I was told she needed to gain weight, but that was because she was actually dropping weight which is not a good thing in a growing child. At the time she had no interest in food and just wasn't eating.
I started adding more fat to all her meals. Vegetables had butter melted on top, cheese was sprinkled over eggs etc. It was like a switch was flicked. All of sudden she was hungry all the time and she has been eating really well ever since. I make sure she gets a decent about of carbs but don't stress if she doesn't get a set amount every day. We do a lot of dairy but it doesn't seem to effect her adversely.
Back to the hospital for a checkup today and she has gained weight and is the healthiest she has ever been. Primal eating really has made a difference for her.
I don't think that every skinny kid needs to gain weight. My daughter is still very slim. But when needed I found adding more calories was a good thing. Now just watch out for my thread "Help, my 7 year old is eating me out of house and home!"
Became Primal August 2011
SW - 84kg / 185lb
CW - 60kg / 132lb
GW - 60kg / 132lb