See here for similar question albeit India not Pakistan but same principle applies
https://www.canadavisa.com/canada-immigration-discussion-board/threads/applying-pr-for-child-from-india.534172/
They need to be back in and remain in Canada in order to sponsor the baby for PR, there is no option to add children post a PR landing.
If they have not met the residency obligation by the the time they return then it is advisable to not try any sponsorship until such time they are back in compliance otherwise it is possible their PR status might be revoked.
Others can correct me but believe in the mean time they will need to apply for a TRV for the baby until such time they can sponsor a PR application from within Canada.