One time as a single parent, I took my children to Canada. I happened to have with me the decree of divorce, and the chidren's birth certificate with me. We crossed and Windsor, but came back via Montreal. The customs official from the US was curious about a single dad with two very young children in the car; I smiled and said, "they are mine,would you like to see their birth certificates. The boy said cheerily, "I'll get them out of the glove compartment, Dad." The fellow said that it wasn't necessary...
I'm not an attorney, but my understanding of divorce law is that until there is a court ordered agreement mandating where and with whom the children should be, that either parent has equal access. You could be very clear in writing a letter to your ex detailing where you will be going, how to contact you, and when you will be back and send it via proof of mailing by the USPS. As long as you honor what you say in the letter, there shouldn't be any grounds for legal problems.
Also see your auto insurance agent to issue you a Canadian Auto Insurance Card.