In Canada, a consent letter is highly recommended when travelling abroad with children. This includes children travelling abroad alone, with only one parent or guardian, with friends or relatives, or a group.
The letter should be signed by every non-accompanying person or organization with the legal right to make major decisions for the child including anyone with custody rights, guardianship rights or parental authority. Government of Canada has created a consent letter template and sample letters on their website, visit https://travel.gc.ca/travelling/children/consent-letter for more information.