I assure you, my son is one of those children people stop and stare at in the stores. People speak to each other about his "lack of discipline" or saying "that kid needs a smack" or even tossing thinly veiled insults at me.
My son is autistic. No, it's not a disorder you can see by looking at him, there are no obvious physical signs. He also has related sensory integration dysfunction, a disorder that makes his brain get jumbled and unable to process certain visual and auditory stimuli. Flourescent lights, the type used in essentially every grocery store these days, are a huge trigger for him. Because of the way his brain works, they are visually like strobe lights to him. He can see the flicker, and it hurts his eyes and gives him headaches. When he was younger, it took only a few moments in a store before he would have sensory overload and be screaming, crying, and flailing. Oftentimes if I could let him down to walk, instead of being pushed in the cart, he could look at the floor and cope much better, however at the 1 1/2 to 2 year old range he couldn't watch the floor and watch mommy to follow at the same time. Leashes saved his life more than once at that stage, and when he was a little bit older we graduated to a transmitter/receiver set that sets off an alarm if he gets more than 10 feet from me, which we still use today, and he also wears at school.
Just try to bear in mind, next time you see a screaming child at a grocery store and feel like saying something rude to the parent about their lack of discipline. You don't always know what's going on, and your 2 second assessment of the situation might not always be accurate.