Vitamins, minerals and fiber comes from the vegetables which are important for growth and a healthy body. Do they eat fresh fruit? If so, then give them extra fresh fruit to make-up for the lack of vegetables. Suggest giving them a daily chewable children's multi- vitamin to give them vitamins and minerals they need. Hoped this helped you.
Perhaps it's time to start hiding vegetables in their food and finding ways to make eating vegetables appealing. I know an adult who won't eat vegetables or fruit unless they're in overly processed food. That adult is reaching 300 pounds and has multiple health problems, including insulin dependent diabetes. The diabetes is no surprise.