Uncle, what is my child's name?

The relationship between your uncle and your children is that between grandparents and grandchildren. So your child should call him uncle. Uncle is your father's brother and the son of your grandparents. Your child is your father's grandson, you are uncle's nephew, and your child is uncle's nephew. So uncle and your children are grandparents, and uncle is your children's great-uncle. They are also closely related by blood. They are collateral blood relatives of the fourth generation.

Four generations of collateral blood relatives, the first generation counts from you, the second generation counts from parents, and the third generation counts from grandparents. Grandma's generation is the fourth generation, and four generations of collateral can get married.