Start by giving them a multi vitamin.
You can't make them eat anything, but you can make them try. They need to take a bite of whatever veg that you are serving. It can sometimes take 50 tries to change their minds. If they eat spaghetti sauce, which is a veg, anyway, try blending cooked carrots, zucchini, etc and adding it to the sauce. As they get older, they will be more open to new foods.