Being that you are both PRs, at least one of you has to be resident in Canada throughout the sponsorship process - which will happen through Buffalo regardless of whether the child is in the States or in Canada. Dependent child applications are not processed within Canada. That doesn't mean the child has to be in the States during the sponsorship process - however, whether or not your child is admitted to Canada (as a visitor) depends on the officer who assesses your entry to Canada at the border. Chances are that a baby who is visa-exempt would be admitted without too much problem, but you should probably be prepared to show that you actually intend to sponsor the child - so they won't worry that you won't - and you can do that by paying the PR application fees prior to trying to bring the child into Canada and providing the receipt when you enter. It will be understood that you can't submit the application until you are in Canada, but you can pay in advance to at least show that you understand the need to apply.
