If your Mom/Dad have multiple entry visas to both countries then no worries. The POE officer may grant both your Mom and Dad another 6 months since they were away for a while (POE officer may or may not reset the allowed 6 months, but then they both have multiple entries and they didn't stay long the first time) but even if the visas were not multiple they can re-enter Canada on the basis of their first granted stay if they only left for the US and not for other countries.
thanks for the response PMM.
Just to clarify again, the max time for any visitor to Canada will be 6 months at anytime right? So my mom can enter Canada to send off my dad who is leaving in Dec and go back to the U.S and then come back into Canada again to leave for India in March?
Untill the 6 months are done I don't have to do anything right?
Thanks.