I sat down with mine and explaimed everything I knew about vitamins an immune system. We drew germs and "soldiers" from our body that fight germs and infections. I then told him that his soldiers need some food and the food they need are the vitamins in fruits and veggies. Then I went on telling him that he needs to eat a rainbow of colors every day. That's how we did it. He now eats even broccoli without any issues! He understands it's for his own good.
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