When my son was between the ages of 3 & 4, he would not tolerate a vegetable on his plate. Unless ketchup is a vegetable.
So we used to buy things like green beans and carrots pureed as babyfood, and add them to his ketchup, with a pinch extra salt, a bissel garlic, a dash more vinegar, and a drop of red food coloring. He never knew we were sneaking him vegetables.
Eventually, he got interested in battered, deep-fried vegges that he could dip in ketchup, so he ate carrots, sweet potatoes, broccoli, mushrooms, Russet potatoes, and green beans. He was very thin at this time, so the “deep-fried” part was not a concern.
Also, at this time, he would eat virtually anything that was a fresh fruit, and he ate eggs and fortified peanut butter and tofu hotdogs, so we didn’t worry about his iron.
We also made him cookies with fortified oatmeal, peanuts, and raisins and low sugar, and brownies that had black beans in them.
His doctor said he was very healthy, and we should write a book on feeding vegetarian kids.
I don’t know how much parents would like ideas that involved “sneaking” foods into other foods, though.
FWIW, our son also drank some Carnation Inst. Breakfast (1 tsp) and 1/2 & 1/2 (1 tblsp) in his milk for a while, because he grew 6 inches from age 1 to age 2, and was in the 95th percentile for height, and the 10th for weight. Now he is above the 99th percentile for both. Anyway, we were really motivated to get him to eat.
DH would sometimes mix whole milk, 1/2 & 1/2, vanilla inst. breakfast, and juice, and call it a “baby smoothie.” The boychik loved those.