^^^This!
I've read that Tom' Sawyer's Island can be a nice spot for a break as well, depending on when you go. There might be people there, but it's away from the noise, the concrete, the stimulating colors etc... In fact, we're planning on bagging our lunch and eating on TSI and taking that as a break.
We've been watching YouTube videos for a couple months now, showing them all the shows, rides, gauging interest. I've even been "simulating" the roller coasters, lol. I sit a child on my lap and as we watch the video of say Big Thunder Mountain, I try and move the way the coaster is - ex. pulling them back and bumping my legs when going up the track, suddenly dropping them forward and to the left to simulate going around a turn etc... On the flip side - lesson learned, don't show them anything that you are not willing to do because if they get it fixated in their mind... well we all know how that will end, lol! Example? I would honestly love to skip Hollywood Studios... but I can't. Because they saw the Indiana Jones video. And every day it's "How much longer till Indiana Jones?" And so... we will be going to HS just to watch Indiana Jones
It's even nice for just us the parent to preview stuff, so we can gauge if it's something we even want to allow them to try. Ex. Imagintion with Figment at Epcot is a sensory-heavy ride. It's a mild ride as far as speed, bumps etc... , but could be sensory overload for some kids - it combines heavy visual stimulation in a dark environment along with smell and noise. So does Stitch's Great Escape at MK, It's Tough to be a Bug at AK. We will not be doing any of those, just because it will be way too stimulating. My son is VERY sensitive to visual stimulation in certain lighting. Also, the last two (Stitch and Bug have sudden surprise elements, that could really be upsetting for some kids who don't do well with sudden loud noises, or sudden touch).
The other thing to consider that hasn't been mentioned is where you're going to stay. Not sure if you're planning on staying on-site, but most of the Disney value resorts are sensory-overload. Even the rooms are bold colors, the resort is bold, with characters, activities, noise etc... all around you. There is no space in an open hotel room for the child if they need a break away from you, some alone, quiet-down time. Nor is there anywhere for you to go if you need a break, or if they're in meltdown mode (and you need to plan on meltdowns!) etc... I do think some of the moderate and definitely many of the Deluxe resorts are "calmer" both in atmosphere and color scheme.
Also consider if you stay on site in a room, you are dependent on eating every meal in a restaurant. This may be fine, or it may not work for you depending on your child's needs. Again, the restaurants can be loud, busy, stimulating in decor, overwhelming sensory-wise. As an occasional thing this may be okay... but as your only option?
We chose to stay off-site,and I'm really glad for this very reason. We have an entire house to ourselves, 4 bedrooms, with our own private pool. It is nice neutral, calm colors/decor. It's in more of a neighborhood atmosphere instead of a busy resort. There is space for each of my children to go to their own room to unwind if they need. If they need to shut the door and scream and cry, I can go to the living room for a few minutes to gather myself. Having a house will allow us to keep more of a normal routine - supper at a kitchen table, relaxing in a living room before bed watching a favorite movie, then off to private bedrooms - this is a benefit for us, a well as the kids. Having a private bedroom allows them a normal bedtime, without us having to be stuck in a hotel room with the tv off for the rest of the night when it's only 8:00.
Also, we won't need to eat every meal in a restaurant. Breakfast can be at the house every morning, we can make/bag lunches and find quiet spots to eat them in the park instead of being in a busy restaurant. We're specifically planning our days so that we can be back to the house for supper time, so again, they can eat in a more calming environment, instead of every place being stimulating. Now, we are doing SOME ADRs, yes - Tusker House in AK, Cinderella's Royal Table, and Hoop Dee Doo Revue dinner show. However they're strategically placed on half days, when they have some down time first.
So my rather lengthy point, lol, is to consider your entire trip - not just your time in parks, but consider everything, where you'll be staying, eating etc... and try and make it a place where you can keep as much normal routine as possible, and which can be as calming as possible. If you really want to stay on site, then I would rent a cabin at Fort Wilderness Campgroudn. It gives you a separate bedroom, a kitchen and dining table, living area, and it's a very neutral, warm, calm decor. Also, the general area that the cabins are in are more secluded, calming, away from the hustle and bustle of the resort. We looked at them, and were going to stay there, but it was
much cheaper for us to stay off-site (we're about 20 minutes driving time from MK, but we're only paying $100 a night for 4-bedroom house with private pool and games room, that is reviewed at 5 stars). But there are definitely house-rentals that are closer to Disney, that will likely still cost less than staying on site.