As a Nanny, I ALWAYS discuss rate during my initial phone conversation.
I don't bring it up immediately, but I do make sure it is discussed before I drive to their home to meet them.
I live in CA where gas is $4.50/gallon now (!) so I want to make darn sure I don't waste my time and gas.
Plus, it also makes sense not to waste the family's time as well.
Since you already talked to them on the phone, I would suggest you send them an e-mail as soon as possible just letting them know that you enjoyed speaking to them on the phone and letting them know that you look forward to meeting them on whatever day you will meet them, but that you forgot to discuss rate with them. Let them know what your rate is and ask them if that is within their budget. If so, great. If not, then thank them for their time and let them know that you simply cannot work for any less than that and wish them luck in finding their new nanny!
I have seen many ads on CL where it is listed, "Rate to be discussed during interview."
I always run from those ads!!
Best of luck to you.