OP, I get you. It's frustrating to feel like systems are in place that make it harder to help your kids eat something reasonably healthy and/or what they want, whether that be the food items or the portion size. Research shows that the #1 predictor of how much you will eat at a meal is the portion size provided. Even if you never finish a meal, you will eat more if I give you 10oz of a food item rather than 5oz. And it's not easy or convenient to... What exactly do PPs propose you do? Ask for a spare plate to scoop out some of your kid's meal as soon as it arrives? Ask for a doggy bag pre-emptively? Tell the child to only eat half? This thread has a lot of preaching, including from people who didn't read your post closely, but not a lot of reasonable actionable advice. (Hence all the assumptions that you were on the DDP.)
I hope more people who have children who struggle with their weight and diet or who have their own significant weight problems might respond with more empathy and more ideas that perhaps they have used successfully.
To be honest, I have no particular empathy for the OP.
My son was diagnosed with hypoglycemia when he was 10. Up to that point it was a constant struggle to keep him from tipping over into "overweight" on the BMI charts (he was usually right on the cusp - always a little over or a little under). He would cry constantly that he was hungry, but when I'd offer him a banana, he wouldn't want it. And then I would end up saying things like, "If you were REALLY hungry, you'd eat the darn banana!" (Only I said something a little stronger than "darn".) He was constantly on a calorie quest, and food became an obsession in our household.
When he was finally diagnosed, he got put on a low glycemic diet, in order to even out his blood sugar. (By "diet" I mean an eating plan. We weren't trying to get him to lose weight. Children are almost never supposed to lose weight, they're just supposed to grow into the weight they currently are.)
The first time we visited Disney, he was undiagnosed. The second time, he was newly diagnosed and we had to figure out how to keep him on his diet.
So, with that in mind, here's every tip I can think of....
* Don't order pop! Don't even order him a sugar-free pop. Get him a water. It's free, and it won't shoot his sugars through the roof.
* Your child does NOT need dessert. No one needs dessert. If dessert comes included with the meal, then ask for one of their sugarfree options (usually the cheese cake). If the child doesn't like it, too bad, so sad.
* Feel like treating everyone to frozen treats? No problem. The diabetic/hypoglycemic child gets a chocolate dipped frozen banana. We are on vacation, after all!
* How much the child eats matters a heck of a lot less than what they eat. So, don't get all hung up on controlling portion size at Disney. Instead, keep sit down meals and buffets to a minimum and focus on making healthy choices. (ie, salads, wraps, flatbreads, etc) You don't need to eat in a restaurant every single day!
* Keep a bag of nuts on hand for sudden attacks of the munchies.
* Make sure everyone is well hydrated (thirst can masquerade as hunger).
* Breakfast should be instant porridge with dried fruit and nuts, made in your own hotel room (you'll need to bring your own electric kettle if you're staying in a Value). It's inexpensive, fast, easy, and will get you off to a good start, blood-sugar-wise. Avoid heavy, greasy breakfasts in the food court.
* Eat proper meals three times a day at regular, predictable intervals. Include two snacks between the meals.
* NEVER criticize a child for leaving food on their place. Clearing your plate is not a virtue. "Wasting" food is not a sin. Those starving children in Africa don't give a good gosh-darn whether you ate all of your hamburger and fries.
* And never, ever try to diet at Walt Disney World. You're on vacation! Just like Christmas and Thanksgiving, you'll eat more than you usually eat. You'll indulge. That's okay! You're also being a lot more active than usual, so it should wash out in the end. Well, if your child is confined to a stroller, the only person getting an unusual amount of exercise might be you, but trust me it really won't make a difference in the long run. You're not living at Disney, you're visiting. There's no bad example being set. Your child will not expect Mickey Ears and Dole Whips and Turkey Legs when he gets home.
Personally, I LOVE Disney food. And I was incredibly happy with the lengths Disney went to accommodate our son's dietary needs. I can't think of a single other vacation destination we've ever been to that's so flexible and diet-friendly. (Canada's Wonderland was awful.)