Check the book NurtureShock
out at the library - they have a great chapter in it about all the research about race issues and children. Per the research, what you want to do is great! When we don't talk about race to our kids, they will learn that it's taboo, and make up their own reasons/rationales. Worth reading that chapter, for sure, if you haven't already.
We just read Follow the Drinking Gourd
which is about the Underground Railroad (my dds are 6 and 3). Nice illustrations, and clear. We had a long talk (the six year old and I) about slavery and what it entailed, etc., afterward.
Our library had a book called Cumbayah
(Kumbayah) awhile ago, in the kids' section, which I liked simply because it talked about the Gullah dialect - the illustrations people of all different backgrounds, which I liked too.
I don't know where you live - where I live, the last census showed that something like 99% of the population is Caucasian. There isn't a lot of interaction opportunity for our dds that include people of different ethnicities.
So -- I do and have brought it up in conversation. We have talked about why people have different skin colors or etc. (from a biological anthropology perspective, my background, I've probably been far too detailed in talking about population dispersal patterns etc.
), and that skin colors have no impact on what people can do, and that all people deserve respect and support and compassion, etc.
I would add, at the same time it's probably a good idea to reinforce what you've taught about not commenting on other peoples' bodies, hair etc. unless you're speaking directly to them and know them well (or however you've described that to them) - otherwise, you're likely to have a kiddo commenting loudly about the guy at the Y being African-American or something.
Which some people are understanding of, but others, not so much.
I think (or should I say, hope?) that exposing the girls to books that have pictures of kids from different backgrounds etc., and history too, will help lay a good foundation in our situation .... We recently checked a book that was 'quilts for 50 states' with each quilt square accompanied by a couple paragraphs about that particular state -- without realizing it, I ended up giving Ina a cliff's notes summary of American history, including the Mason-Dixon Line, Missouri Compromise, Civil War, etc. etc. It was also a good platform for beginning discussions too. The book is Quilt of States
, and National Geographic is associated with it. My dd1 is really into history, it might not be as interesting to a kiddo who's less interested in history...