The seme didn’t know where his kid was for like three years iirc? By the time the uke’s “mother” (in quotes because to me she is no mother) finally revealed where she was keeping him, the child was already calling the nanny mom, which the seme corrected right away. But the nanny started immediately trying to act like a wife to the seme as well so he chose to relocate and he couldn’t bring his kid with him since that would mean having the nanny over again and he couldn’t just get rid of the nanny right away because of how attached the kid had become to her. So in reality, the only ones to blame are, firstly, the uke’s “mother” and the nanny for not staying professional and implementing boundaries. She had the obligation to inform the child who his parents were really, even if she had to make up reasons for why they weren’t present with him yet.
It was very cruel of the younger nanny to allow the child to call her mom, though seme is also to blame for being so absent.