While we don’t know how long after the flood that the grandchildren were born, but we know who they were. Cush is the son of Ham, and grandson of Noah.
“And the sons of Ham; Cush, and Mizraim, and Phut, and Canaan.” – Genesis 10:6
As the sons of Ham began to spread out from “the land of Ham”, it is Cush that settles the area currently known as Sudan. It is here that Cush begins to have sons that will also eventually spread out to other areas.
The Sons of Cush (Genesis 10:7-8)