Yes, people with missing limbs can be very nice to kids who are busily embarrassing their grownups with frank commentary. I remember one of my nieces commenting loudly that "that man should be TALL!" when seeing a guy with no legs, and they guy just said yes, I used to be taller, but now I'm not, and that was fine with her. I, of course, was mortified, as the adult in charge of the loudmouth kid, but the fella made me feel a lot better.

It is up to the parents to react appropriately, like No, she isn't hurt, she's different, and isn't that interesting that someone with one arm can do all she does?