Now for clarification.
A US born child, who is UNDER age 16 does NOT need a passport to enter Canada, or re-enter the USA. The adult does need a passport.
The child DOES need to have their US birth certificate with them.
A letter of permission from both parents, and contact phone numbers, is required, should the CBSA officer want to make a phone call to check information. It should contain dates of travel and destinations and return date.
CBSA has a duty to make sure that all minor children entering Canada are NOT being kidnapped or abducted. They take that very seriously, indeed.
Jim B.
Toronto.