I don’t know their situation. I would guess that they’ve picked up bits and pieces of their grandmother’s language, more of a broken good Chinese than a true dialect such as is Ebonics.
Well, not to belabor the point, but “. .our grand kids don’t speak good Chinese” is then not grammatically correct, just as ‘don’t speak good English’, or ‘don’t play good baseball’ are. Which I found ironic considering the topic.