Many times, the problem is not what a child is eating, but simply how much.  Small children have small stomachs and need to eat more frequently than adults.  Children may need to eat five or six smaller meals, along with snacks, each day. Whenever an underweight child feels hungry, feed him or her. While sprinkling in snacks as needed, make mealtimes regular focal points in your child’s day.  Teach him or her that eating is both important and enjoyable.  If mealtime seems like an annoyance or afterthought, or some sort of punishment (such as sitting until you clean your plate), then children are less likely to be enthusiastic eaters. Make mealtimes a regular routine.  Turn off the TV.  Make eating and enjoying the focus. While your kid may need to put on a few pounds, you might benefit from losing a few.  Even if this is the case, your eating habits should not be as different as you may think.  Eating a variety of nutrient-rich foods is essential for the underweight, the overweight, and everyone in between.  Children learn by watching you.  If you regularly try new foods and make healthy options, like fruits, vegetables, and whole grains your first choices, they are more likely to adopt these habits. Making junk food a rare indulgence will benefit both of you, whether you need to gain or lose weight. Like healthy eating, exercise is more often associated with weight loss than weight gain.  When paired with smart eating, though, it can be part of a weight-gain regimen.  For older children in particular, adding muscle mass is likely to increase weight, and is always healthier than adding body fat. Exercise can often stimulate the appetite, so try encouraging physical activity before mealtimes and see if that helps.

TL;DR in Vietnamese:
Cho trẻ thiếu cân ăn thường xuyên hơn. Biến giờ ăn thành một thời điểm quan trọng. Đưa ra một ví dụ tốt. Khuyến khích tập thể dục thường xuyên.