A big part of this is "knowing your kid." Granted, WDW is an experience like no other, but you've probably had a glimpse of what overexcitement and overtiredness can do to your child.
We were lucky because DD was not prone to meltdowns. She had a few in her day, but usually only because too many factors were thrown in, such as she was also sick, Grandma kept wanting to bring her friends over to show off DD even though DD was too tired and sick to be adorable, there had been a recent time change.....these played havoc with her, we were staying at someone else's house, she had to stay up past her bedtime, etc. It tooks LOTS of those added together to bring on one of her rare meltdowns.
Her pattern is to run herself ragged with unending energy and then pass out as soon as the motion stops. She could do WDW from 8 a.m. until 10 p.m. or later without pausing, but as soon as we get into the car or a bus, she will be asleep and DH will have to carry her comatose body back to the hotel. For my part, I think we got off lucky.
Had she been prone to meltdowns, I'd never keep her out that long. We have friends who can predict the arrival of a meltdown. Some do things to avoid it (naps, breaks, etc.) and some just keep forging ahead right into the path of the tornado.

Some little kids can only take so much assault on their senses. And unlike mine, most get worn out. It is the rare child who can go from early morning until late at night without negative behavior popping up in some way. And that's to be expected. It's asking too much of them.
When I see a parent dragging a kid in the process of a meltdown (mini or grande) through WDW or someplace similar, going on and on about how this is their vacation (and oh no....how they've spent $X on it) and they child WILL have a good time and quit whining, crying, etc. it makes me shake my head. Usually, the kid has been "set up for failure" if the parents are expecting perfectly behaved children. Also, kids that age have no concept of what a vacation costs. They'd be just as happy at the kiddy pool.
And if the kids are melting down, yes people are going to stare and/or shake heads. They have no idea if the kid is autistic and the parents are doing the best they can
or if the kid is being dragged around for 10 hours by parents who will not give the kid a chance to rest like a 4 y.o. needs to, thereby making everyone around him miserable
or if junior throws fits on a daily basis and his parents have just learned to tune him out and expect everyone else to do the same. And, as someone said, they want to eat a meal without listening to a screaming child for the entire time.
It's tough for everyone. I really saw very few meltdowns. The ones I stare at are when I hear parents speaking very harshly to the child and it's clear they have unrealistic expectations of a small child. That irritates me. That doesn't necessarily mean they should ignore the meltdown. I've seen parents deal with them calmly and effectively. And yes, removing the child from a restaurant or a show is called for sometimes. No one wants to be the one with a fit-throwing kid, but it happens sometimes.
A year or so ago, we were with a family who turned out to have the meltdown fit-throwers for kids. For the first time,
we were the group that people stared at in a restaurant thinking, "Those parents need to get those heathens under control." I was mortified, as was DH and DD. And you know what? I can't blame the other patrons. I had never seen anything like it. It happened everywhere we went. No autism to attribute it to, just a complete lack of discipline.

And yes, I am sure "we" messed up the evening for other diners near us on more than one occasion. Subtle hints to behave fell on deaf ears. Oh well.....WE were mortified, as I said. The parents were blissfully unaware that anything was amiss or that anyone was in the least irritated.

At some point, I believe the waitress would have tipped US to leave.
