B.,
I wish I had something comforting to say or school advice to give. The insane truth is that is about the best you are going to find. I am a single mother, and had to pay $105 per week. The school was not even close to the high end. That was with my income level discount. Extremely expensive and it certainly makes it difficult to raise your children and be able to put food on the table.
I have a couple of other ideas that are more in the rhealm of realistic possibilities.
1) Bring in a couple of kids her age, or a year older. So you can put an add out for in home child care. You will be bringing some money in the home, as well as providing a structured environment that you can watch your children with the other children, and thus, get the same result...socialization and education for your children and others. Preparation for school...your way.
2) Arrange for play dates with other mothers on this site or other places that you have mommy interaction. Many mothers are on the same page, and you just haven't been made aware, or thought about the idea. This way, you do not have other children in your home, unless you have the play dates at eachothers homes. Some places I would recommend meeting at would be: childrens museum, library, parks- public or neighborhood, YMCA- or other sort of places of the sort that you may have accessable (YMCA does do a sliding scale. You would most certainly qualify for a discount with so many of you in the family).
I hope this has helped. I think you may have been at a dead end idea before. I wish you luck in accomplishing what you are set out to do!
A.