Usually, there is not much different for one-trip or round ticket in term of the price.
So, if you parent eventually need to come back China or plan to visit China later, you better book a round ticket. Also, if your parent hold visitor visa, you better book round ticket for them.
It is unlikely you can book a one-trip ticket from Canada to US, or it can cost more than 1000 dollar. Though, it is usually a few hundred for a round ticket.