They are starting to believe she's not coming back

Wow that sucks.
I guess you have to think in terms of support. Certainly in the short term.
Figure out what it is you need most and compile a list of needs.

When you have that I suppose you have to find out if anyone around you could help with these issues.
That will be tough because most people are unreliable when you need them most. So if you have friends or family that might be able to help, try and keep them onside by not worrying too much about how they do things.

Then comes the hard bit.
You all have conditions that require input from specialists.
That's medical; social; educational and probably financial.
So you either have to do a bit of research and see if there's help available via a charity or group local to you connected with disability or education or psychology and make contact with the intention being to try and support your family's needs yourself. Or you have to bite the bullet and involve a social worker who might well assess your needs and decide on things that take 'control' out of your hands completely.
However well intentioned that's a whole other level of stress.

As for their Mother... This is the bit that worries me the most.
Primarily because she has all the power in this scenario.
What are the chances she'll return at some point with new partner/people around her that make her think she's now capable of taking on her children (you having done the hard work raising them)
You may need legal advice to protect you and them from future problems

Those poor kids. I'm so sorry she did this to them and you're left with the fallout. At least they've got you. 🤗

I'm positive you'll make sure they know it's okay to have and express all the feelings they're gonna feel about it, whatever they may be. Make sure they know if they need/want to talk to someone about it that isn't you, that you've got their back and will help them find the support they need. [Especially your girl whom I saw in another post already seems to have figured out to find a lady friend to help her get her feminine stuff she might be embarrassed to ask you for]

Your poor kids, they don't deserve that. 😔

Damn. That's friggin awful. I'm so sorry.

What about filing a missing person's report with the police, unless you have an idea where she might be? If not, its unfair to your kids if you do nothing.

How old are they?