There are PFD's that are toddler-size and they should wear them whenever they are near the water, as well as when they are in the canoe.  They get used to them quickly.  We never took our children to the canoe country when they were small, but we have a lake cottage, so we have had PFD's for the grandchildren since toddler age.

I think it is also a good idea to begin young to take a child into the water, just for the sake of comfort, so that they aren't terrified if they should fall in.  Floating around in the PFD is fun, and good practice just in case they should ever end up in the water by accident.