In case you find cousin relationships confusing, here are the basics.
All grand children are first cousins. All first cousins have the same grand parents. (Except some are sibllings).
All great-grand children are second cousins. All second cousins have the same great-grand parents. (Except some are first cousins and siblings).
If your grand parents are someone's great-grand parents, then they are your first cousin once-removed. They are your kids' second cousin.
If your grandparents are someone's great-great-grand parents, then they are your first cousin twice-removed.
It keeps going but that's probably enough to get the idea.