I agree with @Sporkasia when she says teaching kids to eat healthy from the start is the best way to help them stay healthy and get all the vitamins they need, but anyone with kids knows this is not always possible.
When she was young, my older daughter refused to eat most vegetables and my wife was determined that the girl was going to eat healthy. I can't tell you how many times my wife said, if you don't eat your vegetables you're going to sit at this table all night. My daughter was stubborn and she probably would have slept at the table rather than eat those vegetables.
Anyway, my wife gave in before my daughter, and my daughter didn't have to sleep at the table. In the end, we decided we would do the best we could to get the kids to eat healthy, but we knew we were coming up short in some areas. So we gave them vitamin supplements and did what we could to push the vegetables and fruits. There are some cases when vitamin C supplements are needed.