If they want a multiple entry visa - they would need to apply for a brand new visa. It's possible they may receive a multiple - or they may receive a single again.
When someone applies for a visa, it's CIC's choice whether they issue a single or multiple entry visa. In your parents' case, they've chosen to issue a single entry visa. Single entry visas tend to be issued when CIC feels the applicant doesn't have sufficiently strong ties to their home country.
