Okay so I need help. My 4-year-old lost it completely in the middle of a grocery store last week because I would not buy a particular cereal. Full crying, floor sitting, the whole thing. Every person in that aisle had an opinion on their face 
What do you all actually do when this happens? Looking for real answers not just breathe deeply.
Oh DataPioneer the cereal aisle. A classic 
My son once did this exact thing over a box of dinosaur-shaped pasta. Not even something he had ever eaten before. Just saw it and decided his whole life depended on it.
What works for us is immediate distraction before it fully escalates. Like the second I see the lip quiver I pivot hard. I point at something random nearby and say something like wait look at THAT, completely overdramatic, and about 60 percent of the time it breaks the tension long enough to move us away from the offending item.
The other 40 percent of the time we just ride it out in the corner of the aisle and I pretend to be very interested in a can of tomatoes until it passes. Solidarity to all parents in aisle seven. 
The floor sitting is a whole thing and nobody prepares you for the moment you have to decide do I pick them up or do I also just sit on the floor.
What genuinely helps us is getting physically low before it explodes. Like crouching down to their eye level, talking quietly, not matching their energy at all. Something about a parent suddenly going calm and quiet when a kid expects loudness tends to confuse them just enough to pause the spiral.
Also snacks. I know it sounds like bribery but a small snack pouch appearing at the right moment has saved more grocery trips than I can count. I have no shame about it.
Okay NerdNode44 the dinosaur pasta story is sending me The completely arbitrary nature of what sets them off is genuinely baffling. My daughter once melted down because the clouds looked wrong. THE CLOUDS.
My go-to is giving her a job. I hand her something to carry or ask her to help me find a specific item on the shelf. Something that makes her feel like she has a role. It does not always work but when it does it works fast because suddenly she is focused on the task instead of whatever she was upset about.
I think the key is giving them somewhere to put that energy. Meltdowns often happen when kids feel like they have zero agency over anything happening around them.
The giving-them-a-job thing CloudKernel11 mentioned is so real. There is actual research behind it too. When kids feel some sense of involvement or choice, even a small fake one, it reduces the feeling of powerlessness that often triggers the meltdown in the first place.
A trick that works along the same lines is the two-choice method. Instead of saying no to the cereal, you say we are not getting that one today but you can pick between these two. You are still the one who decided which two are on the table but they feel like they made the final call. Works better than a flat no, at least some of the time.
Does it always work? No. Sometimes they want the dinosaur pasta and no amount of psychology is getting between them and that box.
I want to be real here: sometimes nothing works and you just have to get through it
I had one of those moments in a busy shopping centre last Christmas. My son was three, exhausted, overstimulated, and had been promised nothing specific but somehow expected everything. He sat down in the middle of the walkway and refused to move. People were literally walking around us.
I sat down next to him on the floor. Full cross-legged, right there in public. A few people stared, one older woman smiled at me like she had been there herself, and my son was so confused by me joining him on the floor that he stopped crying and just looked at me.
Sometimes matching their energy in the most unexpected way is the only move left.
CodeSphere12 sitting on the floor with him is honestly genius and I am filing that away for emergencies
My practical tip is the whisper method. Instead of getting louder when they get louder, I drop my voice to almost a whisper. Kids who are in full meltdown mode are so used to adults getting louder that a sudden whisper makes them go wait what did you say and they actually quiet down to hear you. It feels backwards but it works more often than raising your voice does.
Also I have started keeping one specific small toy in my bag that only comes out during emergencies. It is not a permanent part of the toy collection. It is purely for public meltdown situations. The novelty of it buys me enough time to redirect.
The whisper trick NexaByte43 mentioned is one I use too and the look on their face the first time you try it is priceless. They fully stop mid-cry to try and figure out what is happening
For me personally the biggest thing was learning to recognize the signs before we even got to the store. My daughter has a specific face she makes about 10 minutes before a meltdown and once I started noticing it I could usually head things off. Tired, hungry, or overstimulated before you even walk through the door means the odds of a public meltdown go way up.
So now I try not to do big errands after school, right before meals, or when she has been in busy environments all day. Obvious in hindsight but I did not connect those dots for a while.
Bitnova55 that pre-trip awareness is something more people should talk about because so many meltdowns are preventable if you catch the setup conditions early enough.
That said sometimes life does not let you plan perfectly and you are in the cereal aisle at 5pm with a tired kid because that is just how Tuesday went.
In those moments my strategy is to get outside fast if I can. Fresh air, a change of environment, stepping out of the store even for two minutes does something to reset the situation. The store itself can feel overwhelming for small kids, bright lights, noise, too many things, and just leaving that environment even briefly helps bring the temperature down.
If leaving is not possible I aim for the quietest corner I can find and wait it out without adding more stimulation.
I feel like every parent develops their own version of the meltdown toolkit over time and it is very trial and error at the start
Mine is pretty simple. I narrate what is happening in a calm flat voice. Not coaching them, not saying stop crying, just describing it. Something like I see you are really upset right now. You really wanted that cereal and we are not getting it and that feels bad. Just naming it out loud without judgment.
A lot of the time kids are also frustrated because they do not have the words for what they feel yet. When you put words to it for them something shifts. It does not stop the feeling but it makes them feel heard, and a kid who feels heard is slightly less likely to escalate. Slightly. Results may vary
TechSphereX the narrating thing is something I read about in a parenting book and thought sounded ridiculous until I actually tried it in desperation one day and it worked Still feels a bit theatrical standing in a store quietly describing my child’s emotions out loud but here we are.
One thing I want to add that nobody has mentioned yet: your own face matters a lot. Kids read your expression first. If you walk into the meltdown looking stressed or embarrassed because of the people around you, they can feel that and it can actually make things worse.
The audience of strangers in the store does not matter. Most of them have been there or will be there. Focusing on just your kid and blocking out everyone else in the room helps you stay calm and that calm transfers to them faster than any trick does.
Okay adding my contribution which is going to sound unhinged but stay with me
I sing. Not a lullaby, not a calm song. I just start singing whatever comes into my head, usually something totally random like the theme from a cooking show or an old pop song. Loudly enough that my kid hears it but not loud enough to cause a second scene.
My son finds it so embarrassing that he immediately wants me to stop, and in trying to get me to stop he completely forgets what he was crying about. It also makes me laugh which breaks my own tension, and when I start laughing the whole energy shifts.
Will it work for every kid? Probably not. But for mine it is the fastest circuit breaker I have found. ModTechLab is right that your own energy sets the tone, and nothing resets my energy faster than choosing to be absurd about it.