Your first option, and probably the best one----if the child isn't affecting you in any way, just ignore them. You don't have to react at all. This is the option most people choose, and it's fine.
However, if you feel you must or if you want to react, one handy trick is to find the parent and to smile at them. Smile that understanding smile that says "Hey, we've all been there" or perhaps "I see your interesting and intriguing child" or even "I can tell your child is autistic, and I just want to give you a smile of support". Any of those smiles, those looks, will generally be welcomed by a parent or caregiver.
But what if the child is bothering you or annoying you? Well, first of all, judge how long you'll really have to be in their presence. If you are on a subway, or in a store, or grabbing some takeout, or in a park, are you really going to have to be around that child more than a few minutes? Can you find it in yourself to think "yes, they are being a little annoying, but I know their parents are doing the best they can, and they live with that child all the time. They are dealing with a disability, and what would I have them do? Hide the child away?" Think those things, and then go back to idea #1 or idea #2. Ignore them, or be supportive and smile.
What if you have to be with the child for longer? Well, then, think about whether the parents have any choice about bringing their child to be in your presence. Are you in a doctor's waiting room, and they are too? Or are you at a playground with your children, as they are with theirs? Then ask yourself---what if the child had a physical deformity that upset you to see? Would you say "Why would they force me to be in the presence of that child? They should have kept them hidden away. They shouldn't have taken them for medical care or to a public recreation area" Then change the words physical deformity for mental disability, and think how you think about yourself for thinking that way.
Now let's go over the rarest of situations. You are at someplace you paid to be at, trying to enjoy yourself. You are at a restaurant, or a movie, or a museum. There is an autistic child there, screaming or laughing or talking loudly. Their parents don't seem to be succeeding in quieting them. They are bothering you, and you feel like your outing, for which you paid good money, is being ruined. You feel like giving those parents a piece of your mind, or at least asking them to leave.
This is where it gets tricky. But for a minute, put yourself in the shoes of the parents. Perhaps they haven't eaten out as a family for years. They have decided to give it one more try. It isn't going well. They have paid for their meal, and they are going to do their best to get through it. Perhaps the mother is aching to take the child out to the car, to once again sit outside so the rest of the family can enjoy the meal and so strangers aren't annoyed. Or maybe it's the museum. The child loves whatever the museum shows, but the parents have resisted taking them there, knowing that their behavior might bother others. Finally, one day, they give it a try, bracing themselves for the situation.
There's no good answer here. The kind thing to do, the compassionate thing, would be to go way back to the first two ideas. Ignore the child, or smile and support the parent. Or go ahead. Complain. It isn't fair to you. You'll make the parents extremely hurt, or extremely angry. They will learn their lesson. They will learn to keep that child home. But keep a little statistic in mind. They say one in 88 children is autistic. You might be the next one with autistic child. Or grandchild. And what goes around, comes around.
I'll end with a thank you, a thank you to the probably 95% of people who ignore or smile. Thank you for giving me the courage to give Janey a full life. To the 5%, I am sorry. I'm sorry if my child has disturbed you. I can sincerely say I hope we don't cross your path again.