I think your explanations of how God created everyone is sufficient. To over explain makes you look nervous and gives your children the idea that there is something to be worried about. I'm white from the south. My husband is Puerto Rican from Illinois. He has cousins married to black women. Our church has every different kind of nationality. At times we have seen someone in a store that was different from ourselves (like a couple in which the husband was a midget...I'm sure that's not politically correct these days). My kids have asked questions...I just told them basically what you have said to yours and they are just fine with that.
I was raised a good bit in the south, but my father grew up in the north. I grew up in the military exposed to all kinds in all kinds of places. Racism exists every where in every culture...and not just racism..prejudice of every form. All of us has some kind of hangup from our childhood or youth. It is impossible for anyone to have no inkling of distrust or dislike of some group somewhere. Now days it's gotten so crazy with the political correctness that people don't even want to speak the truth about an individual for fear of what someone might think. You can't say a thing about President Obama without someone calling you a racist. We are all way too tense.
Teach your children that we are all different. That you approach everyone with kindness and respect regardless of what they look like, where they come from, how much money they have or don't have......
Set the example for them and they will learn from you. If they ask you a question, then answer it, but don't go into all the gory details and fill their heads with notions that make them ponder whether stereotypes are true.
And by all means teach them that hatred, fear, and plain meaness can come from Anyone for Any reason.
The most ridiculous thing I ever heard was a student telling me that I gave him an F because he was black. When we got into a discussion about racism he explained to me that he couldn't be a racist because he was black.
I had an uncle up north tell me the south was just full of racists right after I heard him make comments about stingy Jews.
And I heard my black classmates call Mexicans beaners in AZ.
Then I have a cousin that went to an all white highschool, but they looked down their noses at each other depending on their name or financial standing in the community.
It's all based on fear of the unknown or the assumptions made out of ignorance. Teach your kids that God loves us all, created us all, and died for us all...and who are we to telll God He had it all wrong.