My preschool is predominantly white middle/upper class. We do have a few children of different ethnicities come through the center and it never fails, the children end up having "you're not like me" discussions. I monitor and intervene when it's necessary, but I try to let the children lead the way. Sometimes it's as simple as "Your skin is black and mine is white" "Yup, that's just the way I am!" "Let's go build a bridge" "Ok!" and sometimes it's been "You can't be my friend because you don't look like me" "Uhhh, what?".
I always feel awkward bringing it up to parents ("So, he had a good day..played in the sandbox, counted to 20, had a discussion about racial equality in the modern day preschool environment...") and I don't know why. It's not like no one involved doesn't know/see that they are definitely outnumbered in the class and we all know how kids talk openly about these things.
Most of these discussions have centered around race, but we do have some kids with special diets, ability levels, skin tones, family situations, etc. Would you, as a parent, want to know if these types of discussions/situations happened (even if they didn't escalate into a big deal) or would you rather not know?
btw, I hate these questions when I sound like an idiot because typing it out makes me feel stupid, lol.