The derogatory term "anchor baby" refers to the child of noncitizens who gets citizenship from being born in the USA, and who is then used to establish citizenship for their parents. Barron Trump is the son of a US citizen and gets his citizenship from his father.
Melania Trump got her citizenship through her marriage to Donald Trump. While she wasn't a citizen when Barron was born, he was not the basis for her residency, her leave to remain, or her citizenship. She would have been just as eligible whether she had had a child or not.
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