Hey nuts can be given only in moderation. You can use dates, Anjir and raisins as sweetener like you can soak them and boil them and use that water as sweetening agent to their food. Also if the child is above one year you can give grated soaked almond and small pieces of dates. Also sweet potato, avocado, pumpkin, paneer are all source of good fat and help in weight gain.

Yes you can give.. you can give all dry fruits like that or even after; soakin at night.. banana milk paneer .. add ghee in his food.