my 42 month old is underweight, what can i do?

I have a set of twins and a single child. the other twin is 30lbs and my 2yr old is 28lbs, they all eat the same things. Now my other twin(Kaelin) is only 24lbs she hasnt gained weight in almost a yr. She eats fruits, vegetables, and sometimes she'll eat more than the other two, but she wont gain weight. we give her whole milk, and she has also been on pediasure b4 aswell, but the pediasure didn't do much. i was wondering what else could explain why she isnt gaining the weight she needs?

[Renee Radenberg]

Her weight is below the 5th percentile and her height is between the 25-10th percentile for her age. Suggest giving her Pediasure or milkshakes (milk, ice cream, and fruit blended) or yogurt smoothies (yogurt, and frozen or fresh fruit blended) in place of milk for meals and snacks for additional calories and protein for weight gain. Here are some additional tips for extra calories/protein: If she likes pasta, then add meat or cream sauces to the pasta instead of plain tomato sauce or plain pasta. Add dry milk to oatmeal, mashed potatoes, gravies, creamed vegetables, and creamed soups (add additional meats to soups). Use all fat dairy products for now until she starts gaining weight, then start using low fat in later years. Hoped this helped.