Indeed crossing the border into Canada can be an issue - my wife was asked for proof of permission from me to bring the kids across this summer. Her utter surprise and shock at the question "Does your husband know you're taking the kids out of the country?" was enough for the officer to let her pass, but he said she could be barred from entry in the future without a letter.
When the kids travel with their grandparents, we send them with a letter and state that they have our permission to escort them, and they also have permission to make decisions regarding medical care in the event the children are injured or sick.
We enter the dates of travel, and notarize the letter. They have not been asked for the letter when flying domestic with the kids yet, but have not been out of the country, either.
I think it's a pretty simple note. Dates of travel, permission to take the child out of the country. I'm not sure whether you need medical consent authorization, but that might be nice too.
Notarized, and you should be good to go.