My youngest has a birthmark on her face, and I don't mind at all when people stare or ask questions. We also hear some rude comments, or weird people who will gush, "Oh, the poor dear, oh it really looks bad, oh it looks like she got hurt, awwww". I fell bad that those people are such total morons, but so far I don't mind since my daughter is 2 and doesn't know better yet.
That said, I ALWAYS made the rule since the kids were talking, that you don't talk about what people LOOK like. Period. Unless it's nice. You don't say they're "big" or "skinny" or ____race color (my daughter would say, "look, mom, a Japan person" if she saw someone Asian after the news coverage for the earthquake. I just kept reminding, "don't talk about how people look, you just say 'lady' or 'man' or 'child'. If you are looking at them, dont' stare, and be prepared to say hello and smile if they see you" etc. Then I would explain the many differences later, but they knew not to say something in the moment. My husband and I live by that too, we don't say "look at the ____ lady", we say, "look at the lady with the blue hat". Just while the kids are sooo young, we want to set that habit. As they get older we can be a little more lax.
At other times, during reading books and stuff, I make sure to explain there are thousands of different ways people can look different, but they all have feelings, and you never make people feel bad for how they look. We go over it a lot actually. I really want them to understand that behavior is wrong when they see other people do it as well as not doing it themselves.
I had a great teacher who once said. "It's great not to say something mean about someone, but it's even greater not to think it." We're trying to model and instill that while the kids are young.
Thankfully we saw a man in the airport who was badly burned and disfigured. I was ASTOUNDED when my 3 yo son just said "hi" to him in the security line like anyone else, and then asked me LATER what happened to him. ASTOUNDED that he followed the rule even in a really extraordinary case. That rule (no pointing or asking or talking about what people look like) has saved us some awkward statements a few times.