What is a fair pay?

I work as a full time nanny and take care of 17 mo. old twin girls getting paid $14.00/hour.I have been with this family a year and a half. The family will expend in May with another baby. How much of a raise can I ask for? I live in the Sacramento,California area and work in Folsom and both parents work.
I am CPR certified and have 13 years of experience working as a nanny.