Be prepared no to be allowed to return to US.

Usually, the B2 visa is multiple entry visa for half year.

And the I-94 is also for 6 months. Please check the validation date of the visa. If it is expired, your parents need a new visa to reenter US from Canada.

I doubt it will work to get around the renewal application. So many people who have failed on extanded application never think about and try that way.

If you never heard any successful example, I suggest you not try to be the first one. It is too risky and it could become a big hassle to you and your parents.