third cousin once removed
Your son and your nephew are first cousins to each other. Their daughters are second cousins to each other.
Cousine
Your daughter and your first cousin's son are second cousins to each other.
They are second cousins
Your first cousin's daughter and your son are second cousins to each other. Your second cousin's daughter and your son are third cousins to each other. Your third cousin's daughter and your son are fourth cousins to each other. And so forth.
Your first cousin's daughter and your son are second cousins to each other. Your second cousin's daughter and your son are third cousins to each other. Your third cousin's daughter and your son are fourth cousins to each other. And so forth.
They are second cousins
Your sister's daughter and your son are first cousins to each other. Your sister's daughter and your son's daughter are first cousins, once removed to each other. Your son's daughter and your sister's granddaughter are second cousins to each other.
I think the kids would be second cousins
Your nephew and your daughter are first cousins to each other. Your nephew's son and your daughter are first cousins, once removed. Your nephew's son and your daughter's child would be second cousins to each other.
Your daughter and your second cousin are second cousins once removed to each other.
If by cousin you mean first cousin, your daughter and your (first) cousin's son's are second cousins to each other.