I'm trying to figure out if there's a DNA test or tests to prove the grandchildren below had the same grandfather.

The grandparents had two children, male and female. We're certain the grandmother was the biological mother of both children, but would like to prove the grandfather was the biological father of the two children. Only grandchildren are living now:

GRANDFATHER: UKNOWN(DECEASED)
Grandmother: Known(deceased)
Child: Known Male(deceased)
Grandchildren: Known Male and Female(living)
Child: Known Female(deceased)
Grandchildren: Known Male and Female(living)

Does anyone have an idea of how to go about this? Any advice would be greatly appreciated.