If you are asking, "What is the relationship of your uncle's son and daughter to you?" they are your first cousins.
If you are asking, "What is the relationship of your uncle's son's daughter to you?" she is your first cousin once removed.
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Your uncle's first cousins are also your parent's first cousins. A daughter of a parent's first cousin is your second cousin. If you are talking about your uncle's second cousin, the daughter would be your third cousin. and so forth.
That would be your cousin by marriage, considering of course that the child is not your uncle's.