The Torah says that after Kayin had a son he built a city and named it after his son. Perhaps the gezeirah for Kayin to wander around for the rest of his life applied only to Kayin and not his descendants. So once he had a child he built a city for his wife and children (and some of the other people who had been born by then) to live in, so they could stay settled while he continued to wander.
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