I agree with Quintilius, although seems harsh, but you were prepared to pay it when you chose to, but just because you are now unable to pay it, it is too much. So many people pull out and whats the point of any terms and conditions if no one abides by them.
They are just doing what they should be doing. Did you fill them in to your circumstances about a year ago when your husband first lost his job? have you kept them informed all the way along this past year? If not, you suddenly spring it on them that you are leaving, when most people by now have already got their nursery places for September, I suspect they will have plenty of places left for September.