It's always better to prepared. This is especially true when you are traveling outside of the country with your child without the child's other parent (or legal guardian or when a child is traveling without either parent). My husband, Connor and I all have current passports and NEXUS cards, but if only one of us is traveling internationally with Connor, we make sure to have the appropriate documentation saying the other is aware of the trip and "consents".
Many believe that it's required for when you clear customs at your destination (Or pre-clearance as the case may be) and yes this is true, but it's equally important when you are returning home.
We have been told by Customs and Immigration officials in Canada and the United States that it makes it much easier for them (and for us) when we are able to provide a signed and stamped official (notary/lawyer) letter. Passport Canada has sample consent letters on their website which include the information required.
You may not be asked to show it, but it's not worth the risk of missing a trip because you don't have it. For further information consult Passport Canada, your airline and your destination country.