A Better Alternative
All young kids are curious when they see a person who looks different, whether it's a child with a disability or a grown-up who's obese. I've observed (even while being observed) that children blurt out what's on their mind not only to quench their thirst for knowledge but also because they're worried that the same strange condition could happen to them. Their poor parents usually don't know how to react, and so they stifle their kid's questions in order to spare themselves embarrassment. That mom at the Empire State Building may have had the best of intentions when she p'revented her son from saying something that could have offended us, but she sent him the message that Ethan and I were strangers to be feared, rather than individuals who were worth getting to know.
What is the best thing for a parent to do? Having been on both sides of these situations, I think you should let your kid embarrass you. That's right. Rather than engineering a plan to avoid an awkward encounter, give her the chance to explore life's anomalies naturally. The key is to trust your child and have the guts to allow the scene to unfold, regardless of the outcome.
If your child sees a man in a wheelchair and proclaims, "Mommy, what's the matter with that man's legs?" you may feel mortified. But that man knows he can't walk. It isn't a secret. Instead of silently mouthing an apology to him and slipping away, I'd suggest that you walk over to him and introduce yourselves. While your child may be shy and prefer to hide behind you, she'll see you engaging the man as a neighbor and a potential friend. I can't promise that this approach will be well received in every instance, but it's worth a try. By making this effort, you will be showing your child that the man is a "someone," not a "something."