This is a terrific conversation. What I intensely dislike about statements like "you're just putting the Chinese kid in there for diversity points" is that it assumes writing white characters is a psychological, sociological, intellectual, and emotional default, and that making any other choice with a character automatically means the author is DIVERGING FROM THE NORM. That makes me very, very upset, because for me and my kids, and for countless other parents and children in the world, the norm is considerably more complex and nuanced.
I write mixed-race characters because my kids are mixed. I write about thoroughly Americanized kids of Asian descent because that's what my experience was growing up. In my first book, my culturally assimilated characters go about their business without giving a shred of thought to their ethnic identities, because that's not what the book is about. In the manuscript I'm working on now, my culturally assimilated characters DO question and explore their ethnic identities, because that IS what the book is about, at least partly. Those choices reflect real-life, real-world, emotionally authentic experiences, and they're every bit as valid as making all the characters white by default.