I think my parents' disapproval of my interracial relationship has influenced the relationship itself
Why do you think that?
I think my boyfriend and his kids have now poisoned my relationship with my parents.
Your boyfriend and his kids are not at fault for the way your parents think, and it is not okay for you to internalize you parents disapproval of them. Letting your parents get to you will destroy your life.
My parents are close minded. I can never live my life like this.
You need to live your life and not be concerned about what others, even your family, thinks. What they think of you is irrelevant. Give them a choice: either accept your interracial relationship or stay out of your life.