You can renew their visa online. it should be done 30 days in advance of the expiry of the current visa. once your application is filed, your parents will go on implied status( a legal status in Canada). IRCC may take 1 month to 3 months to renew your visa. however, your parent's status in Canada will be considered a legal status until their visas are renewed. you should ask for renewal for 5-6 months to avoid any delays which may cause due to the slow process of NDVO.My parents file got transferred to NDVO at the background checks stage at end Nov. They have been here on visitor visas since 9 Aug 2019. So their 6 months stay will expire on 8 Feb 2020. I have taken them to the US twice but passport was never stamped as usual. I am taking them for one week holiday to Punta Cana (Dominican Republic) in mid-Jan. As they will re-enter by air, I believe their 6 months stay will be revalidated from that day. Hope my assumption is correct.
Another option is when you cross the border on the car while return you can request immigration officers to extend and explain the situation they have authority to extend visas. one of my friend's parents' visa was extended on their request at the border for 1 year.