First, they JUST got engaged, and already the groom's mother is researching venues? If she's doing that, then she plans to pay for it!
So, get the kids to breathe, and you try to do it as well. Have a sit-down with the daughter and her fiancé, and ask what they have in mind. Ask WHEN they plan to get married, and that will give you a basic timetable. You can gently ask if they've had discussions about budgets and wish lists and so forth with his family. Discuss what the kids themselves are prepared to save and contribute, if anything. Do you know for sure that they have a set idea of sharing expenses, or did your daughter just tell you this because they had some hypothetical discussions with the groom's parents?
Have you met them? Do you have a good relationship with them? You can invite her and her husband for coffee with you and your husband. Don't invite your ex (your daughter's father) and you can explain to them that he does not participate financially in anything and has not for many years. Try not to badmouth him too much, but say gently that he wishes he could do more and sometimes gets caught up in his desires and isn't as practical as most people would wish. You can say, gently, that you have shielded your daughter from this for 20 years (or whatever it is) but that she's no longer a child and it's important that everyone be realistic. Verify what they are thinking about (don't assume the kids got the story straight or that the groom's family was operating on full and accurate info), and after they respond (important!), them that a 1/3, 1/3, 1/3 division among groom's parents, bride's mother and bride's father isn't going to work. Tell them you cannot foot his third of the bill, and you don't want the children exposed to losing their reception hall or not being able to pay the DJ because Daddy didn't come through. It would be heartbreaking and embarrassing to THEM.
I would, if I were you, set a figure with your husband of what you can contribute. You have extreme medical expenses for some children, are owed 20 years of child support, and did not even attend family weddings due to expense. You can decide that you have X dollars to contribute, which should be roughly what your other children got when they got married. If you need a year to save up, fine - tell the kids that. If the kids want to get married in 3 months, there's not going to be much money. If they want it in a year or two, there will be more.
You can ask the young couple how they want to do this. If they want to do the whole thing themselves, fine. But why is his mother out there window shopping before these discussions have even been held?
I think it would be great to invite the parents over to YOUR HOUSE for a little champagne (not high end) and appetizers to celebrate the young couple. And let them see how you live. They can see your special needs kids, and the reality of your circumstances. Don't apologize, don't grovel, don't do anything to belittle yourselves. You are carrying a super-human load, one that any decent person would admire.
Please resist your inclination to make everything perfect for everyone else in the world while taking all the weight on your own shoulders. You don't have to do a "woe is me" meeting, but you shouldn't fake it either to try to appear more affluent. You are who you are.