In casual speech, you can call them "cousins" or "nephews" and "nieces". Formally, they would be your "first cousin once removed". Your cousin's grandchildren would be your "first cousin twice removed." If you and your first cousin both have children, those children are each others' "second cousins".

Personally, I call them horrible. Just a joke! They aren't that bad.

Just call them cousins.. don't count if once or twice or thrice removed. We all came from the same place anyway.. We're all cousins I guess. They are your cousins, not niece or nephew either... just cousins.

In most circumstances, I would just say cousins because it's easier. If it's relevant to know the exact relation, then they would be second cousins. My children are the second cousins of my cousins (the children of my aunt).