You kind of assume that just because a child is picky (ie has strong preferences) that they are all unhealthy. My kid will not touch mcdonalds (or any processed) nuggets with a 10 foot pole, likes some fries (but not most), won't eat 'soft' white bread or jelly (does like peanut butter in moderation but not enough to make it a meal - actually prefers sunflower seed butter... go figure!) That said sometimes when you are in a strange place with lots of very strange foods, some go-to things can make life a whole lot easier. Just because someone is looking for that on vacation does not mean that it is all they feed their kids at home.
I'm also not sure why it is irrelevant that kids have special needs. I think more and more we understand that the "weird little kid who is really smart but doesn't get it socially" of a generation ago is the aspie kid of this generation or that the kid who just couldn't control himself or sit still of the previous generation is the ADHD kid of this one. Not every kid who doesn't behave as you think they should is doing so due to bad and overly permissive parenting and I have found it is generally better to be empathetic and nonjudgmental of other parents rather than telling them everything they are doing wrong.
Mine is on the autism spectrum (what would have been aspergers before they simplified the DSM) and has sensory issues that go with that, but I rarely bring it up. Why should I? I don't feel the need to constantly excuse or apologize for my child - if someone is going to be intolerant I don't wish to shame them into being tolerant. I especially love going to Disney because there are so many kids of so many walks of life and they treat kids so well there... I never feel pressured into explaining why my kid dropped his milk or why he needs to eat a certain way. All kids are accepted for who they are for the most part there - at least by the CMs if not by all other guests. And I have posted questions asking about my "picky" kid... that's enough info for the sake of a forum about WDW.
I have found that buffets are great with mine because he can try new things with no pressure. No pressure that mommy just spent $x for a meal he won't eat, no pressure that if he doesn't like something he can't try something else. I learned the hard way that applying pressure to my child to get him to eat things did more harm than good and created a lot of anxiety I had to spend years undoing. Now at 7 he asks to try things and I am always proud of him for trying even if he doesn't like it... and he is still picky as all hell. Proteins have always been tough but sliced proteins at buffets are generally fairly bland when unsauced, have no weird lines and no heavily seasonings - if they are crusted the seasonings can be cut off, and also are texturally more pleasing because they are thinly sliced and not huge chunks of meat. That plus some kind of starchy side (rather than making mac and cheese his whole meal) and he does pretty well there. No he won't touch fruits or veggies (although recently has started being ok with apples so I am super proud of him!) All of the Disney buffets have enough kid choices that he feels comfortable trying new things but there is plenty to eat if nothing is a winner. The best advice I got was to take all pressure off and just keep offering, enjoying yourself with meals and offering at every opportunity with no pressure.
Good luck and have a great time