If she isn't on benefits and the partner is happy funding this lifestyle is it really an issue?
Obviously it's ok for you to say no more financial help from yourself, and if you are resenting it then do, but if they are standing on their own two feet and the children are well cared for, clean, fed and loved, then it's a lifestyle choice not a shortcoming.
I live similarly to her (although I am younger with less children and won't personally choose to have more than my current 2). I am happy. Since deciding not to try and do it all (look after children, the house, and have a career) I am much happier in life. I don't have to worry about putting my children into the care of others and whether they are being taken care of properly.
My partner is self employed, and I sell produce at the gate (we don't have a farm, we rent a cheap house in a very low cost area with 3 acres which is my hobby garden, but it mostly feeds my family, makes a small side income to pay for trips out for the children, helps me feel like i am contributing and gives me something to do with the children)
Together our income is only 30000 a year, but we manage and live well.
I guess what I'm trying to get at is that your daughter is unlikely to find meaningful work if she struggles with processing information and has never worked. She may never be employed. The only problem I see is that she seems unhappy. Is she unhappy because of her situation or is she looking unhappy because she feels you are judging her lifestyle?
It wouldn't have been unusual going back even 50 or 60 years so why look down on her for it now?
If she is unhappy with her situation maybe encourage her to do a little bit of work for herself - make or grow something for sale, get into reselling, but otherwise she doesn't need your concern.