Or when someone dresses up as a 1920 gangster, just always happen to Italian...
Perhaps you would like to give us all a 'hierarchy of the races' so we can all judge who punches up or down? You can call it the Rassenkunde des deutschen Volkes.
You are an acceptable costume. Face it. But context matters. The reading kid is not singled out, nobody is making this negative. It could also be perceived as a celebration 'hey I want to be like my adopted asian sister, she is cool'.
Live and let live, and give context if the kid is asking questions. But don't censor because you feel like a special snowflake
See these examples
https://www.culturalsurvival.org/sites/default/files/nmai-sp...
http://www.slate.com/content/dam/slate/blogs/browbeat/2013/1...
https://risetowin.org/assets/img/what-we-do/educate/resource...