I agree with previous posters - if she asks then fine. If not, well she's either figured it out herself or she isnt that bothered. Kids tend to just accept things the way they are.
I was never interested in the fact that I was mixed race - it didnt really cross my mind. It wasnt particularly common where I lived when I was little either.
In think I only ever got called a name once (called a chinky) when I was about 6 - that was the only time I ever asked about it. My nana told my dad who got out a map and explained that my mum came from Thailand and they look a bit different there and sometimes people dont understand why other people look different so call them stuff etc etc....
Oh no - I tell a lie - I got called a paki when I was about 19 by some lads who went past on a bus. I just thought this was amusing actually (being half english half thai) Duh!!