When you are hired as a nanny, you have the option of negotiating your salary, even if you work with an agency. The nanny agency will place you in a position, but you are free to negotiate your work schedule and pay with the family. You will not want to sign a contract until you have come to an agreement concerning your salary.
Note, If you are a nanny currently in other country and wish to work in Canada, there will be no negotiating because employers will only sponsor on a wage recommend by law.