"Why is that man black?"
"Why does that person walk funny?"
Curious, observant children often put us in a spot when they ask innocent questions that they don't realise are inappropriately phrased.
These are the 3 things I'd bear in mind if my young child raises questions about diversity that is not within earshot of the person in question:
1. Acknowledge my child's curiosity.
I know when something is difficult to discuss, the easier way out is to brush it off. But this may confuse your child. Your child wouldn't understand why it is taboo or may think there is genuinely something wrong with that person.
By acknowledging your child's curiosity, you are showing the willingness to be open. So I'd say things like "I see you want to know how he got his skin colour. I wonder too."
2. Address the question for what it is rather than what I think it is.
I try to be mindful of my inherent biases. Children may be pointing out the obvious and not necessarily passing a judgement.
I do what I can to expose my children to people from various cultures, and show them how I interact with them; I gather books about famous people of all gender, race, physical attributes from multiple disciplines. I believe the more we read and learn about the passions of different people, the less my children will associate skin colour to a set of societal biases.
3. If my child made an insensitive remark, explain how that could make someone feel and suggest a suitable way to say it.
Children may not have the right vocabulary. They may make remarks that we know to be insensitive. For example, they may blurt out, "Why is that person walking so weirdly?" in public.
If so, you may want to apologise to the person and say something like "I'm so embarrassed and sorry she said that. I'd have a discussion with her when we get home."
Quickly, but calmly, explain to your child that it is not nice to say someone is weird. It hurts someone's feelings/makes the person feel sad.
If you don't have answers to the question straightaway, say, "I'm not sure, but let me find out more when I get home".
I use facts first to address questions and turn it into a collaborative inquiry process. So, for example, I might explore the human body with my child -- bones, muscles, nerves, to try to understand our make-up. Then address the specific observation my child has made with what we've learnt.
We don't always have the right answers, but the important thing is we are willing to educate ourselves together.