Your nephew's child is your great nephew or great niece depending upon sex.
Your nephew's grandchild is your great great nephew or great great niece, depending on sex.
He would be his great great great uncle
== == 'Girlfriend', or depending on her standing with your son, your 'grandchild's mother'.
They would be your step-grandchild.
"barnebarn" - meaning "grandchild", Norwegian has no term for granddaughter.
The children of your grand nephew (or great nephew) are your great great nephews and great great nieces (depending on whether they are girls or boys).
It's not really a word, but a slang term. The correct term is grandchild and it's one word.
Yes, grandbaby is one word, as is granddaughter and grandson.But it is an informal term, the formal term being grandchild.
Your brother's sons are called your nephews, and his daughters are called your nieces. Collectively, you can refer to them as your nieces and nephews. If you want to be more specific, you can say "my brother's children" as a general term.
Biologically, the grandchild is a step-grandchild to the grandparent that had no children. In matters of the heart, however, the grandchild is whatever the grandparent feels it is.
Solution
The plural of grandchild is grandchildren.
Analytical definition