Racism is a negative attitude to someone based on their race. This isn't even displaying a negative attitude, so far as I can see -- although if an adult asked it I'd definitely call/think them ignorant and/ or insensitive, as I hope would most people.
We're in a period where people are now starting to think very carefully about what they say about and how they treat other people based on their differences. I think this is, in the long run, a far more desirable thing than not caring about it and just being rude, prejudiced, insensitive and yes, even racist/ sexist/ *ist. The sad off-shoot of that is that some people do get overzealous and that is particularly unacceptable at this age. The appropriate thing to do is to underline to the child that it's not the done thing to ask such questions, rather than to jump on their backs. All the same, children can be cruel and that too should be dealt with rather than just ignored.
The balance has in recent years swung as I say from a time when you had to accept persecution to a time when persecution is sometimes seen even when it doesn't exist; hopefully it will settle somewhere in the middle in the long run.