A girl about the age of three approached me, pointed to an Asian boy across the way and said, "that's your kid." I said, "actually, that is not my kid. But that one is." And pointed to my daughter who was at a water fountain. The girl looked back and forth, stared blankly at me as if to say "what are you talking about?" then ran off.
My daughter doesn't look very Japanese, so I'll give her that. But I've actually run into this a lot teaching and being around white kids and they actually think all Asians are related. Especially if there is one Asian adult and a child--to them, it's a no brainier that we should be family.
My tactic has been to sit on my hands so that I don't punch them in the mouth. They don't know. Yet.