Grandchild
A child shares his lineage with a grandchild. The child passes his lineage down to his child and then that child’s child is the grandchild and between them all a lineage is shared.
Look at this family tree. See the child, and then the grandchild. Trace the lineage with your finger as you observe.
そん
まご
None
The son (そん) of a son is a grandchild. In this case, a grandson. Easy.
Just remember that a son is all you really want. You need a son to carry on the family name and the lineage. So make sure that your child and your grandchild are both sons.