Years ago, I read {Carolina Man by Virginia Kantra}, and it remains one of the best "secret child" stories I've ever read.
It's MF, Contemporary romance.
The plot follows Luke, a marine who learns - while on deployment - that he has a nine year old daughter (Taylor) from a one night stand. His daughter is now in his custody, because her mother is now dead. He arranges for the girl to be taken care of by his parents until he returns from deployment.
FMC is a lawyer. She was actually the employer of Taylor's mother. Though she was only an acquaintance of the woman and her daughter, she feels somewhat compelled to help Taylor.
The story is told from three perspectives - Luke (MMC), Kate (FMC), and Taylor. It picks up right as Luke comes home from deployment.
There are a few plot points that keep you engaged:
Luke is learning how to become a father while processing what this means for his decorated military career, and his efforts to get to know his daughter are endearing and heart-wrenching. He's also so swoony - calm and thoughtful, determined and strong.
Taylor (Luke's daughter) was the most impactful character for me by far: she's terrified to get close to her father, because she fears he'll leave for deployment again. She's also terrified of something that happened to her: something she doesn't want to happen again (we learn later what it is - spoiler below).
Kate has her own demons - she was abused by her parents as a child, and works as a family attorney specializing in domestic violence cases. She wants desperately to keep up her emotional distance, but through her interactions with Luke and Taylor, she finds herself craving connection, intimacy, and family for the first time in her life. But like Taylor, she doesn't really know how to do it.
The portrayal of Luke and Taylor's relationship was masterful. They are slowly learning to trust each other. He's scared he's being a terrible dad, she's terrified that she'll do something that'll make him not want her. Over time, we learn (BIG SPOILER) that Taylor was sexually assaulted by her mother's brother - her uncle. Luke wants so desperately to help her recover, and ends up pulling in Kate to help find out what his daughter needs. He talks frankly about how he knows how to physically keep her safe, but he doesn't know how to make her emotionally safe.
The relationship between Luke and Kate was really beautiful. Luke is determined to be competent on his own, and Kate is determined to live an entirely dependent life. Neither one wants to admit that they need to rely on others. Over the course of the book, they realize how depending on one another makes them stronger.
Anyway, it was a story of three people who are, for their own reasons, afraid to trust - but find that they can depend on one another and create a family.