you need to 簽加拿大簽證

If you stay beyond 30 days then you have to get a US visa to come back unless you have valid entry visa, which in your parents case is not a problem because they have multiple entry visas.

30 days is for those visa holders who no longer has a valid entry visa such as F1 (eventhough their I-94 is still be valid), then unpon returning from canada or mexico for a short visit, they can return US without first obtaining another valid entry visas. It is called automatic revalidation. If you are still confused, google the term.