If by cousin you mean first cousin, your daughter and your (first) cousin's son's are second cousins to each other.
They would be 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.
I think the kids would be second cousins
Your daughter and your sister's son are first cousins to each other. Your sister's son's daughter (her granddaughter) and your daughter are first cousins, once removed.
You and your mother's brother are first cousins. There is a generation difference between your son and the daughter. That makes them first cousins once removed.
Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
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.
Your son and your niece are first cousins. Her daughter and your son are first cousins, once removed.
Your daughter and your first cousin's son are second cousins to each other.
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 daughter and your nephew are first cousins to each other. Your daughter and your nephew's son are first cousins, once removed, to each other.
Your daughter and your niece are first cousins. Your daughter than your niece's son are first cousin's once removed. Your daughter's child and your niece's child are second cousins to each other.