For what it's worth, I have never been questioned critically when traveling alone or as a couple with an infant or toddler. Probably possession of the child + the child's ID and travel documents (passport/eTA and birth certificate) is more than adequate.
By the way, make sure you have a ticket for the infant- it is required for international flights even when the babe-in-arms does not occupy his or her own seat. There may be some cost involved for that.