We travel to Mexico and the US for 1 week periods of a time (2 or 3 times a year), so as long as she has a multiple TRV we will be fine?
If your nanny will be exiting only to the US, her single entry visa will suffice.
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A single entry visa allows you to enter Canada only once. This visa may also be used for repeated entries into Canada from the USA or St. Pierre and Miquelon provided you do not enter another country. Such entries must occur within the time validated for your stay in Canada."
She can apply for a multiple entry visa at a Visa Office outside Canada.
http://www.cic.gc.ca/english/information/applications/visa.aspNote:
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NEW: We have started a pilot project with a new application guide and forms for the following visa offices: Buffalo, Detroit, Los Angeles, New York, Seattle and Washington D.C. Applicants to these visa offices should use the new guide and forms. Applicants to all other visa offices should refer to the website for that visa office, as indicated below, for forms and instructions."