What medical reasons could explain why my child has never developed tears?

Your child has tears: I've actually been asked this question before. You have tear glands above your eye and a tear duct in the corner by your nose. Every time you blink, a tear washes across your eye. If you can't make tears, the surface of your eye will dry up and cloud over in a matter of hours. No tears equals no vision. If your child can see, s/he makes tears, whether you see them or not.