My Nephew is 1.5 years old.
My brother and sister-in-law do not know how to control him in public places, especially in theaters.He may start crying/shouting at his will in such places.
What best can be done to keep children calm in public places, especially in theaters?Honestly, in my opinion, get a babysitter for things like a theater outing. Or give up theater night. A theater is no place for a child, much less one of such a young age. Children at that age don't understand reasoning and logic yet, so they can't understand why they need to be quiet, despite their extreme boredom and frustration. Children need attention and a place like a theater is not conducive to that need.
As a parent, you have to let things go for the sake of your children. If that means giving up a little afternoon show, then so be it. They are parents. It's no longer about either their entertainment or their social life. It's about a commitment to a child. They should go to the theater when they have secured a babysitter or go individually while the other stays home or takes the little one somewhere more acceptable for that age.
As far as other places. The disciplinary method they use should be up to them, but they must stay consistent with the approach, and they should never, I repeat never give into the screaming and crying if it's because he is not getting his way.
The kid's 1.5yo, of course he's noisy, even on a good day. They speak loudly, they interrupt people, and they want to share what they think at what adults would consider inappropriate moments. He doesn't know yet all the social boundaries that adults would consider "normal".
Plus at this age, of course they'll throw tantrums "at will". It's their way of showing frustration, or of finding limits.
While you don't always have a choice, you shouldn't take a toddler to just any place. And you should know in advance that they will misbehave. It's on the roadmap, it will happen, don't be surprised if it does. They may even behave perfectly fine 95% of the time and then suddenly make your world hell.
And don't worry too much about it, people are generally indulgent in this regard. Sure, we get pissed and thigh when kids get rowdy on planes or repeat the same thing 57 times, but it's not that bad. Even people who aren't parents are capable of getting on with it without too much effort.
And for those who don't, they probably have a bad day and can't snap out of it, and feel they shouldn't have to deal with your kids. And they're right. But in the end there's just so much you can do, so they'll have to live with it.
As mentioned above, you shouldn't bring kids to just any place. You don't always have a choice: you have to go shopping, have to go work or meet someone, have to go your doctor's appointment, have to go to the DMC or some other administrative office for paperwork, etc... Those are things you can't easily avoid. If you can afford it and have the time to organize it, get a sitter. If you don't, roll with it.
A movie theather seems like a pretty odd place for a 1.5yo to be... except if we're talking a Sunday morning kid-movie showing. But in that case, it's expected that kids will be loud. If it's not for a kid movie, then it seems like the kid shouldn't be here. You don't take a kid that age to watch movies with you, for everybody's sake: yours, the other movie-goers, and even the kid's. Nobody told you fun time was over? You'll go to the movies again in 6 years or so.
Pro-tip: Some countries have a few theaters with "crying rooms". Basicallly theaters with a sound-proofed area where parents can go with toddlers, prams and be as noisy as they want. Not that great a movie experience for you, but you leave other people in peace. I've seen some at least in Australia. Look for family-friendly places.
Of course it doesn't mean they should get a free pass on everything, and that they shouldn't be taught what's acceptable and what's not. But you won't be able to make them just stop right away at this age. So you just need to be firm, say "no" and that it's rude to behave like this in public. Have them look around and notice that people are annoyed by their behavior, and that they keep quiet.
Teach them that you whisper when you are in some places and when nobody else makes any noise.
If you take a kid to a quiet place, bring a quiet distraction.
A picture book, a cartoon on a smartphone with muted volume (works wonders at the doctors' or on trains/planes nowadays, my parents regret they didn't have that for me when I was a toddler!), a etch-a-sketch, a (calm) portable video game, a coloring book.
Avoid tiny cars, action mans, or things that will get them too excited and will make them want to share or express their excitement. Bring games or distractions that favor introversion.
And bring several of those distractions! They get bored fast, or may decide they don't want to play with the toy you picked that day or that they already did enough coloring before. You don't get out unprepared with kids. They'll get bored, and it's your fault if they misbehave, not theirs.
Also, bring food. A hungry toddler is a fussy toddler. Make it foods that aren't too sugary and won't get them too excited, but still things that they like to it. Make them choose it before you leave, so they know you have them and can ask for them.
I just stumbled on this article, which I found fitting for this answer.
I find iPhone to be quite indispensable with kids sometimes. There are great picture book apps that a toddler would enjoy
Why should a toddler remain quiet all day?
They are learning to use their mouths and explore the world. They have little need/understanding of social norms to keep quiet in the theater - see @ChristopherW's answer for how to handle that.
It's kind of worrying when you see someone asking "how can I make a child to do what I want them to do instead of what they keep doing" and expecting instant or long-lasting results. There are many stages to child development when certain activities become possible or temporarily impossible - e.g. a toddler will sit in a high chair to eat dinner, but a 2.5 year old will want to run off and play instead.
