I used to work with a lovely guy who appeareed very disabled - limbs fused in a fixed position, no speech (he would signal yes or no with his eyes) but he was very laid back about staring and comments. He would however find it hilarious when I got pissed off and started lecturing people on his behalf - sorry but the chirstian guy who attempted to 'heal' him while we were out at the pub, well that was just too much for me to take
I know he 'meant well' but ffs
I have told dd(5) about people being born different, and how sometimes parts of people may not work as well as they could, being born without limbs, people being different sizes etc. She was fascinated by a lady in the chemist with dwarfism, however apparently the most interesting thing about 'the small lady' was that she was pushing a blue buggy with cupholders, 'isn't it interesting how there are so many different kinds of buggy, mummy?' 
My advice, FWIW, is that if you look at someone with a disability or in a wheelchair, remember that your eye is drawn to what is different or unusual, the same as when someone has their hair dyed a bright colour, and if you happen to catch that persons eye, just smile, same as you would with anyone else. But staring at someone is just rude unless they are fit 