OP, generally, there is a "base" rate for nanny care that varies depending on the nanny's experience/education, and that "base" is for a single child. An additional child doesn't mean doubling the nanny's pay.
IOW, a nanny caring for a single child and making $500/week is not going to be able to go find a job caring for 2 kids making $1000/week.
If you're paying a decent rate, and you want to retain this nanny after her year is up, consider a pay increase, additional benefits, or something else to sweeten the pot.