That's correct. A PR (not Citizen) can't sponsor a child from outside Canada.
If you are planning to make a life in Canada, why is your wife going back home to have the baby? This way you have to apply for PR status for the baby, and then at some point apply for Citizenship for the child.
If, as Canadian PRs, you have the baby in Canada, then the child will be a Canadian citizen from birth and can get a Canadian passport right away.
But as nationals of your home country you should also be able to get a passport for your baby from your home country.
Seems the most sensible way to do it if you want to live in Canada.
Wayne.