I have been known to have quick come backs and a tongue made of acid. For me it would depend on what was said.
I once had a woman complain that children like mine should not be out in public. I quickly told her, "...my son has to learn how to act in the real world just like your parents taught you but I see they STILL have work to do!
Smile, laugh, and walk away. You can't educate idiots.
If you have a middle finger, make a fist, extend the middle finger while keeping all other fingers down in the fist.
Seriously tho, ignore their first comment. If they continue or, if you are going to continue to be around them, politely explain to them your child's issues. They'll usually feel horrible and understand but, if they don't, you should ignore them and pity their sorry existence.
i find a super direct slightly sarcastic but not "too" condescending answer to be best.
"terribly sorry, my son is on the autism spectrum and doesnt always work with the same rules as the rest of us" then depending on their tone, throw in a "whats your excuse a-hole?) :)
Give them a sympathetic look, shake your head a little, and say "I'll pray for you". Then walk away.