My kids are loud. And curious. And not particularly sensitive. Nor do they give a shit about decorum, politeness or how the world even works. Most of the time, it's like they don't even go here, seriously.
Any time they see something different in any way lately, they've been quick (and loud) to point it out. Either they'll ask a question:
"Mama, why does that mama have such fat legs? You don't have fat legs."
Or they'll make a statement:
"I don't want to sit there, mama. That man looks scary." (It was a woman.)
Then, of course, I mean, what do you do? The subjects of the observation, whether it was shouted or whispered (you know the whisper of a five year old...you can hear it across the room, and, honestly, that almost seems more rude, because in the whispering case, it's as if they know they shouldn't say something, but they do anyway)... Anyway, the subjects of the observations have no obligation to laugh it off, and I really can't blame them for the dirty looks thrown my way. If some jerk kid said something like that about me, I'd be casting dirty looks too.
It's uncomfortable because as much as I want to talk to the subject about it, apologize for my child's accidental rudeness and use it as a teachable moment to show off how enlightened and awesome I actually am, none of that is called for, nor would it be appreciated. They don't owe me explanation time, and they certainly don't owe me buddy-buddy, kids-will-be-kids bonding moments.
These untimely observations, of course, fly in the face of who I am, and what I'm trying to teach them. I continually talk to them about how all people are different and each one is lovely, that looks mean nothing, that it's better to be kind, understanding and helpful than to be pretty, thin or feminine.
Right now, for some reason, they're basically loling at me over this. I don't get it, but I believe (hope) that if I keep at it, they'll get the message that this is serious and important to their world outlook.
In terms of what to do right there in the moment? It's tough.
Gauge your insultee. How pissed are they? (The girls once made a really big deal out of a Muslim woman wearing full hajib, and when I looked over, mortified, I could see in her eyes that she was down to play. So in that instance, I was able to do a teachable moment type thing, and she played along and helped. By the end of our dinner, the girls had decided she was prettier and nicer than me, and maybe she could be their mom. This, of course, is a best case scenario).
In contrast, the woman waiting at the Olive Garden to whom the girls referred as a scary man completely and pointedly ignored us. I felt incredibly bad, but I explained to them in a quiet tone, although I'm sure she could still hear me, if she were caring to listen to me try to handle my brood, that she was a woman and all people looked different, and that was okay and wonderful.
In cases like that, try to discourage follow up questions. Talk to your kids about it more extensively at a later time. Remember, those on the other end sometimes do not want to be your teaching tool. They just want to live their lives.
The fat-legs commentee, on the other hand, shot me a really dirty look, and I cannot blame her in the slightest. If this happens, for goodness sake, apologize! Don't get long winded in trying to explain the question or behavior. They don't care. Honestly. That's just hot air to make you feel better. Apologize straight up, and tell the person you're working on it. Sometimes I take a jab at my own kids because I'm their mom and I can, and I'll say something like, "I am really sorry. We are working to make them into human beings. Progress has been slow." But, again, don't crack a joke if the person is super pissed. They don't have to listen to your bad attempts at humor as you try to lighten the situation.
Sometimes, you just shouldn't make light of things, you know?
Anyway, what I would recommend not doing is scolding your child harshly in the face of such an observation. For the most part, at least in the case of my kids, they are not doing it out of meanness or bullying. They're just trying to suss out the world.
And, honestly, "we don't say things like that" sounds an awful lot like "we don't say things like that in public" to both your child and the person they've just insulted.
Good luck, Captain Awkward. I know we'll all need it.