Growing up and living in the US, I used to think that flat noses & wide faces was the Chinese/Asian look. But, after living here, I don’t know how anyone can hold on to that stereotype. Yes, there may be a large percentage of Asians with those features, but there are also a lot without. I have see Asians with long faces. I have seen Asians with what I’ve always heard described as a “Roman” nose (i.e., large and sharp).
And, as far as the eyes, which you didn’t mention… there are so many varieties of the “almond” shape Asian eyes – big and rounder, small and thin, etc… I used to think that all Asian eyes were the same. Then, as I made more Asian friends in the US, I started to think that Japanese had one basic shape, Koreans another, etc… However, spending all my time looking into Asian faces here (and staring into my bf’s eyes), I’ve seen many who, because of their eyes mostly, I would have categorized as being Japanese or Korean, not Chinese/Taiwanese.
Living in Taiwan has just confirmed to me what a professor of mine once said… “There is more variety within each [ethnic group] than there is between them.”
Perhaps the only reason you “notice” a lot of Asian-looking bi-ethnic kids is because you only truly notice the ones that look stereotypically Asian. The ones that have more caucasian features – i.e., hair color, eye color, etc. – you probably just assume (like everyone else) that they are waiguoren.