Whenever only one parent travels out of the country with one or more children, the required paperwork gets a little more complicated. This is due to efforts in both the United States and Canada of border officers to work together to prevent the abduction of children.
What You Need to Bring
In general, besides your passport, you should bring a Child Travel Consent Letter from the child’s biological parent(s) along with the child’s birth certificate. There's also the NEXUS card, which can expedite your crossing.
Here's what the Canadian Border Services Agency website says about required consent documents:
"Parents who share custody of their children should carry copies of the legal custody documents. It is also recommended that they have a consent letter from the other custodial parent to take the child on a trip out of the country. The parent's full name, address and telephone number should be included in the consent letter.
When travelling with a group of vehicles, parents or guardians should arrive at the border in the same vehicle as the children.
Adults who are not parents or guardians should have written permission from the parents or guardians to supervise the children. The consent letter should include addresses and telephone numbers where the parents or guardian can be reached.
CBSA officers watch for missing children, and may ask detailed questions about the children who are travelling with you."
I have a personal anecdote that illustrates just how seriously US and Canadian border agents take this. A few years ago my kids and I were driving back into the United States from the Canadian side of Niagara Falls. The US border agent asked to see my passport, my children’s birth certificates, and a consent letter from my husband. Then he asked me to open the side door of my minivan so he could look into the back seat. He asked my younger son (age 5 at the time) who I was. Next, he asked my older son (then age 8) for his full name and my first name.
Because the agent was polite and handled it with humor, my kids thought it was exciting and not at all scary, and we were quickly on our way.
While we were able to get on with our trip, the take-away is that border agents take reviewing the identification of minors very seriously. Before a solo parent travels internationally with kids, it's important to get relevant paperwork in order and be prepared to answer a few routine questions. It's much better to be overprepared than underprepared, since you don't want your trip to be delayed or jeopardized because of missing documentation.
You may also find these articles helpful:
Looking for family vacation advice? Here’s how to ask Suzanne your question.