I've brought my oldest son's friend camping in the states for 4 days and he only needed a letter from his mother and it wasn't notorized and it was fine. Sometimes they ask for a letter from my husband if its just me taking our 4 year old across the border but again that isn't notorized either and it was fine.
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The Canadian government says other wise. You are required to get your documents notarized. Whether the customs guards follow the rules is a whole other issue.
I was told by Air Canada that my DD would not be permitted to board her flight to Mexico (she was travelling with my mother that time) unless my mother had the proper documents giving her legal permission to take my DD out of Canada. That was the first time my DD left the country without us so we did as we were told and got a notarized letter, which due to my ignorance on the subject at that time cost me $300. My mother was never asked a single question about my DD nor was she asked to produce any documents.

On other trips that my DD has been on the adults she travelled with were grilled beyond belief. So you just never know who you will get and how much they will go by the book.