If you are dealing with a picky eater, try to give them smaller portions at each meal. By doing this, they won't feel overwhelmed with the amount of food in front of them and if they are hungry, they will be more inclined to eat their entire meal since it is not a huge serving size.
For young kids you want to make eating healthy fun for them. If they don't like vegetables, try cooking them differently or serving them with a sauce your child likes. You can also try cutting foods into different shapes like stars, dinosaurs, etc. so that the kid can have fun while eating healthy.
A great nutrition tip is to invest in a blender. Blenders are great because you can mix a wide variety of foods into one simple beverage. You can throw in a handful of fruits, a scoop of protein powder, and some oats in a blender to create a fantastic and healthy shake.
When you want to gain weight you need to do it the healthy way. When you take in more calories than you can burn, you gain weight. Fat is a common ingredient in our food and has twice the amount of calories as protein or carbohydrate. To stay healthy, you need to get your extra calories from a beneficial combination of nutrients not just from fats.