The US and Canadian boarder do not require anything to be notarized. They do recommend it as if the boarder guard has any suspicions it will ease their minds where as a signed letter suffice but they may question more.
https://help.cbp.gov/app/answers/det...hPaGh5bA%3D%3D
http://www.ezbordercrossing.com/the-...with-children/