The Billionaire Daddy (2012)

Blurb:
After a one-night stand, Lauren’s sister became pregnant, gave birth, left the baby girl with the father and disappeared. But Lauren’s not about to leave her niece with the kind of monstrous man who would toy with a woman’s heart. Lauren steels herself to confront Dade in his penthouse apartment . . . only for Dade to mistake her for his new nanny! This gives Lauren a master plan: she’ll live with him as his nanny in order to find proof that he’s unfit to be a father, giving her a fighting chance in the pending custody battle. But will spending her vacation with this known playboy prove to be more dangerous than she could have guessed?

Source: www.anisearch.com/manga/31231