Taking vitamins as an appetite stimulant is an extremely popular practice among parents. A corollary to this belief is that vitamins can stimulate growth and improve strength. All of these beliefs are pure nonsense.
Some believe that the more vitamins they give to their children, the more these children will grow. But providing vitamins to your children is just like watering a plant. More water doesn't necessarily mean better growth. In fact, overwatering a plant will eventually kill it. If a plant is already getting enough water, an excess of water will do no good. If your child is already getting all her vitamins and nutrients from a balanced diet, additional vitamins are just redundant and potentially even harmful.
Many parents rationalize that because their children are picky eaters, they must supplement food with vitamins to make up for the poor intake of fruits and vegetables. This type of thinking is not only wrong but also dangerous. Taking vitamin supplements does not replace the need to eat fresh fruits and vegetables. Vitamins in pill form are an inadequate substitute for the real thing.
Vitamins do not help your child gain weight. Only additional caloric intake can increase body mass. Vitamins simply help the body function properly. You can compare them to credit cards. Like credit cards, vitamins make chemical transactions in the body easier and faster. Without them, the body cannot conduct business in many situations. However, the credit cards themselves do not make you richer. They just assist the transaction along the way.
If your child is underweight, she needs to consume more foods and calories. Throwing vitamins or other supplements at her won't do her any good.