I worked as a nanny for a child who was severely handicapped for three years. I was looking for a job in a hospital or rehab facility and I came across the job posting on monster.com. When it was time for me to move on, to have my own child, we were able to find a great replacement for me on monster as well.
I would try to post where people may not be specifically looking for nanny jobs, that way you may find someone who is willing to take on more of a challenge. Good luck with you search, if I was still in CA I would tell you to bring them over to my house.