The family has to make the call. They may accept your children but reject the girlfriend and the new baby. You could like present the best argument for it being a safe, stable, loving environment, but even that might not be enough. Polyamory isn't a new concept, but society still doesn't view it is normal or right. I wouldn't push it too much because your husband probably cares about his family and may not want to sacrifice the relationships. That's his call to make. They might be tolerant of it in due time, but that could take months, years, or never happen. If y'all are happy, then that's all that matters.