Basically if they have medical issues I'd say GMS- they are quite affordable and have pretty good coverage for things like prescription drugs.
If they are perfectly healthy- Travelance or Manulife would do- both have good customer service and don't seem to have any hassle with claims.
Take a look which one has the lowest rate:
<a href=http://arbetovinsurance.com/visitorstocanada/>Insurance for Parents</a>