How many years should it take till u neet ur boyfriends other kid?

Been with my boyfriend 6 years meet his son 2 times both his kids live in California I was told I wasn't allowed to meet his daughter Cuz according to the mother there child would be traumatized if she meet me and he Rly not doing much bout it besides arguing over the phone with her bout it Idk what to do in bout to give up and tell him to just move out there and will go our own way but I love him alot