In many cases it all depends on the child. Every child is different. Would I attempt WDW with 3 kids under the age of 3? No way. My child is 5 and this is her 2nd trip to WDW. Much like others are asking not to be judged on why they elect to sit vs. stand on a bus, I won't even begin to think I know why they decided to bring that many young kids with them to WDW.
Maybe they were with a bigger group that got on another bus, maybe one member of that group paid for them to come, maybe (hopefully not) one of their kids is ill and they wanted them to experience Disney.
I find it ironic that some (not all or even the majority, but some) that say they won't give up their seat and not to judge them for it will in turn say why would you even think about doing this kind of a trip with that many kids, in your condition, etc...when the fact is they have no idea why that person is on that bus at that moment, what life events have brought them to WDW, etc...
Why so many people have their panties in a wad over a 10-15 minute bus ride I just cannot understand? Were you not in a crowded park with these same people all day? If everyone that needed/wanted a seat took a cab how long would those lines be? How many threads would there be over the rudeness of the people in the cab lines? If you're going to ride the bus ride it. It's obviously going to be crowded and you might have to stand. If you are sitting and want to offer your seat to someone, thank you and good for you. If you decide you just can't/don't want to then don't and good for you. No reasons need to be given either way. Go back to your hotel room and for goodness sake get some rest and get up to enjoy another day.
I deliberately visit WDW during the LOW season. I am anti-crowd. And I still want my own car and avoid the bus like the plague. But sometimes we still get stuck taking it.
What I am saying is that WDW is not the easiest place in the world to navigate and manage when you are healthy and ablebodied, let alone when any complications exist. Right now a friend of mine is considering a trip to WDW for the first time. They have FOUR kids. She wants me to help her plan. My first piece of advice will be that she cannot plan the same way I do. What my family of 3, with one older child can do IS NOT what her family of 6, with four children ages 2-8 can do.
For one thing, we can fit comfortably in even a small hotel room. Not them. If we had to, we could take the bus. Taking a 2, 5, 7 and 8 y.o to WDW for your first time, no doubt with a stroller or two in tow, is NOT going to be fun if you try to take the bus. They don't even have enough hands to keep one of each child and the strollers. To me, getting on a bus under those circumstances is walking right into misery, so I am advising her against it and to instead rely on a car. Wrangling 4 little ones every morning to go to a restaurant for breakfast? Doable, but maybe staying in a condo right next to Disney would be a better fit. (Avoid the buses even more!)
Sure, you can bring 3 tiny kids under age 2.5 years to WDW, but you put yourself at the mercy of luck and on buses, someone giving up a seat for you. You cannot count on that. Some nights, you need an army of angels just to get out of MK after the fireworks. As a mom, it's one thing for me to put myself at risk for falling down, getting knocked over, etc., but when it comes to my little kid, (kids if I had more than one) it's just not worth it. IMHO, that is.
I am not comfortable depending on the mercy of other people, so to speak. Especially not when I'm on vacation. Most especially when I am trying to keep 3 little kids safe. Not worth the trouble to me. YMMV and clearly, for some it does.
That still doesn't mean that my reaction upon seeing something like a husband and wife with 2 strollers, a tiny infant, and a 1.5 and 2.5 y.o won't be, "What were they thinking?"

Are they entitled to chose to visit WDW? Sure thing? And I'm entitled to wonder what made them decide NOW was the time to go. Yeah, someone could be dying, but that's probably rarer than hidden disabilities, ya know? Or maybe they just expect others will get up and offer them their seats because they have 3 little kids............