Depends. Is best to get vitamins directly from a healthy diet with colorful salads, fruits and vegetables. Adding vitamins to food can be tricky - e.g., Folic Acid can not be converted to the usable form of folate (folic acid) in a percentage of the population. The build-up of unused Folic Acid can have detrimental health effects. If protein filled is adding something i would stay away from excess processing.
Too much vitamins... If you are over worried about foods for your kids, so much as to think about or already doing what you mentioned, that is the key to your kids being overweight or even obese. Healthy life style means the harmony between sleep 8h/d + exercise 60mn/d + eat balanced meals (fruits + veggies as sources of vitamins and fibers + not greasy meats for protein or better is vegetal protein (soya-beans..).