To help your child gain weight, try increasing their portion sizes at mealtimes, especially for starchy foods such as bread, rice, pasta and potatoes. Alternatively, if your child finds it hard to eat larger portions, try increasing the energy density of your child's meals, until they have reached a healthy weight.. Best way is to Homemade protein smoothies. Drinking homemade protein smoothies can be a highly nutritious and quick way to gain weight. ... Milk. ... Rice. ... Nuts and nut butters. ... Red meats. ... Potatoes and starches. home made Protein supplements which will help you to have a great boost for the weight gain.