First of all,

to our little corner of the DIS! Everyone here is really nice and helpful!
I agree with everything my good friend
@gap2368 said; an additional adult for support/help will be super important (and probably the first thing we would have told you to plan on - so you are already ahead of the game!)
A triple stroller might be good to have - the main things to know are that not only can you get the "Stroller as a wheelchair" tag at Guest Relations (where you will go to ask for the
DAS for your kiddo(s) ) but you will also want to make sure that any stroller you buy/rent/borrow will fit within Disney's maximum measurements, as stated on
this page at the WDW official website, under the heading "Prohibited items"
Remember that "boogie boards" (or similar devices that create a step for a child to stand on, on a wheelchair or stroller) will also not be allowed - Disney has now changed the language on the Park Rules to reflect this a little more clearly ("Devices should be single rider")
Food allergies typically are not a huge problem at WDW; they easily handle "common" food allergies, and even some not-so-common ones as well. In addition to having a list of those allergies for your servers, remember that buffets will always charge you for all kids over the age of (I think it's 3?) even if they can't eat a single thing off the buffet - so be sure to plan meals ahead accordingly if you think there won't be anything he can eat.
In "normal" times, Disney is great about keeping all of the menus updated for every single food service location at WDW. Right now, it's my understanding that most of the food service locations (especially at the hotels) are running on smaller, more limited menus until they get their "sea legs" with the phased reopening. But, you have plenty of time to plan, so that won't be a problem, and I believe that eventually we will see a return to the complete menus we saw previously.
You don't say - are you planning on staying on WDW at a Resort Hotel? Or are you planning on staying offsite? That will have a direct impact on your planning, and things like afternoon naps (*much* easier to take the bus back to a Disney Resort Hotel for an afternoon nap, and then return for some evening fun). Plus, some Resorts have "alternate" transportation that the kids might really like, including the Skyliner, the Monorail and boats of different types (depends on which hotel you stay at).
If I were in your shoes, I would also start working with my littles *now* on things like setting expectations and "practicing" for Disney World - for example, at the grocery store when you are waiting in line to check out, you can "practice" waiting in line at Disney World. This summer "practice" putting on sunscreen so we don't get sunburned at Disney World!" And any other things that you might want to work on between now and then. Planning out as far as you are is smart, because it gives your entire family lots of time to get ready.
The DAS will help when you are in the Parks by allowing your party to wait outside the main queue for rides, but it has no bearing whatsoever on your time at any food service location, any bathroom location, Disney Springs, or the Resort Hotels, so you will still need to be prepared to possibly stand in lines for things like breakfast at the hotel, or waiting for buses, waiting to buy souvenirs in the shops, etc.
I can tell you are feeling overwhelmed, so what is your greatest concern right now? I know that you are obviously worried about the kids eloping when they have the adults outnumbered, but you also have strategies for that identified (leash/harness & triple stroller), so what is the next thing we can help you work on?
I know it may not seem possible right now, but believe it or not, you are not the first single mom to use a mobility device with more than one child who has their own special needs at Disney World. You can do this - and we will all be glad to help you plan and get ready as much as we can!
