Thanks for your question.
Your child requires more than just vitamins to achieve a healthy weight, your child needs a balanced and varied diet more importantly. Most times, being underweight results from having too little essential nutrients in the diet.
A balanced diet includes foods based on starchy carbohydrates including pasta, rice, and potatoes, a good amount of fruits and vegetables, with protein source from lean meat, fish, eggs, and beans, and some milk and dairy products. All of these have to be in adequate amounts in your child's diet to attain an adequate and healthy weight.
You may visit your GP for a referral to a dietitian to provide a diet program which contains the appropriate combination of these foods for your child. Furthermore, these foods, especially the fruits and vegetables are very rich sources of vitamins needed for your child's growth and development.
I hope I have answered your question.
Best wishes.