Your great uncle's child would be your second cousin.
I think technically it is your first cousin, once removed!
Second cousins, are cousins of cousins.
Your great uncle is your children's great great uncle. They are his great great nieces and great great nephews.
Their great-great uncle
Your great grandparents brothers and sisters are your great uncles and great aunts.
Your aunt is your child's Great Aunt, so your child is her great niece or great nephew.
second cousin once removed
your cousin
your uncles daughter would be your cousin
Your uncle is your child's great uncle and your grandchild's great great uncle. Your grandson is your uncles great great nephew.
He or she is your great-niece/nephew
WHAT relationship* is your Gramps to your great granddaughter? pretty tricky but (distant sisters one off)
The child of your great great grandfather's brother is your first cousin, three times removed.
Cousin. Your parents brothers and sisters are aunts and uncles. and the aunt and uncles children are your cousins.
Your child and your nephew are first cousins. Your child and your great nephew are first cousins, once removed.
Great great aunt/uncle. To your niece, you are the aunt/uncle, to her child you would be the great aunt/uncle. So you would then be the great great aunt/uncle of her child's child (her grandson)