Also, to prevent something like this happening in the future you could always ask for a written review every six months from any future employers. This way, you have a paper trail of positive feedback even if they turn on you in the last second in the event that you decide to leave.
While I think this is a good idea, I just want to put out there that a good number of employers would still pass on you. The reality is that someone may be willing to write a form letter or even a mid point recommendation when they are not ready to let someone go but are in fact less than thrilled with their performance. If I interviewed someone and the MB was giving a negative reference or no reference then the previous letters just wouldn't make too much of a difference. Most people wouldn't want to try to wade through the drama and details or who was right to wrong when there are plenty of nannies with positive references.