It is a good reason to invite your parents to join your weddying. Just go to the Cityhall to get the letter for the ceremony.
As your parents have been Canada and they have a good records in CIC, as long as they have enough funding to support their travel, there is little possible not to issue a visa to them.
Good luck! If you don't invite your parents to your wedding, the visa officer may be surprised when you apply the family reunion.
