I just read an entry on a blog where they were discussing embarrasing things children say. Most were freakin' hilarious, but I have to admit some bothered me quite a bit.
Speaking as a mom of three - 12, 16, & 16 - I have some experience with this. My kids have said and done many funny and embarrasing things. However, I can't ever recall a time when they pointed out a person's disability or excessive weight in a negative or derogitory way. I can't say the same for many of the moms on the blog I visited. What shocked me even more is that the moms admitted to finding the negative comments funny. Or at least they must have because they admitted to laughing at their child's comment(s).
I don't know about any of the moms that visit here, but I have always told my children to not talk about people with excessive weight or a visible disability. My mom did this with me and my sister also.
I've taught them to not stare at people who are different than them or the people in their lives. I've taught them not to make comments (any) about their appearance. If they have a question about someone, they could say to me "Mommy, I have a question" and I would address it when we were out of ear-shot.
I'm all about children checking out their surroundings and noticing that everyone is different. What I have a hard time understanding is why a parent laughs or participates when their child is inappropriate with another human being. I know little one's don't always understand the right and wrong things to say, but momma sure does and momma certainly can set an example by not responding inappropriately.
Just my two cents.