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).
He would be his great great great uncle
Your grandchild and your niece are first cousins, once removed.
You are his grandchild.
The grandchildren would be the parents, aunts and uncles, and first cousins once removed, of the great grandchildren.
Your first cousin's grandchild is your first cousin, twice removed.
You and your uncle's great grandchild are first cousin, twice removed.
Fili and Kili are his nephews.
they are cousins
The grandchild of your great grandmother's sister is your second cousin, once removed.
Yes, they are his nephews.
They are your nieces and nephews.
You brother-in-law is uncle to your children, so his great nephew could be the son of your child, also called your grandchild, or the grandson of one of your siblings who are not married to your brother-in-law, and thus your great nephew as well. But your brother-in-law can also have great nephews who are not related to you at all.