L.T.
My husband and dd are skinny. (I am not) They are spaghetti....I am penne pasta. It must be a metabolism thing. I took DD to the ped because she wasn't even on the weight chart for a 10 year old her size (54 inches). She's not sickly or anything. She has a lot of energy, too. Ped said feed her healthy food and leave it alone. When she hits puberty, it might change.
My husband said his mother used to make him drink several malts or shakes a day to help him gain weight. To this day..he won't even look at a milkshake.
Good luck.