There are lots of parents in all over the world got visa to visit their children in Canada and Travel in Canada, but most of time, due to current policy, each time the visitor can stay in Canada only up to 6 months, no matter the visa is single entry type or multiple entry type. The 6 months period was given by a a Canada Border Services Officer (BSO).
When visitor arrive at the port of entry in Canada, a Canada Border Services Officer (BSO) will stamp your passport or travel document and record the date by which you must leave Canada. The BSO will also note any terms and conditions related to your visit. There may be times you wish to extend your visitor status or change the conditions of your visit. – from cic.gc.ca
So, if your parents who are visiting you want to extend stay in Canada, you can apply to extend for them either online or use mail package:
1: If you apply online:
2: If you apply by mail package:
– Obtain an application package.
– Read the instruction guide.
– Complete the application form and attach the necessary documents.
– Pay the fee and get the necessary receipt.
– Submit the application form and documents.
Please note each person need $75 CAD fee.
More detail please read cic official site.