You have different options. As Parker24 mentioned, you'll have to sponsor your baby for PR. There is no inland sponsorship for children, so you will have to do the outland process. However, at least ONE of the parents needs to be in Canada for the sponsorship process, as PR are not allowed to sponsor from outside of Canada.
So - you could prepare the application now already ( download the outland forms and get familiar with it), and then one parent returns to Canada, establish himself and sends out the application right away, and the second parent stays with the child and waits - outland for US citizen is quoted at 12 months ( + stage 1 to add, currently 50 days), but it's usually faster ( for a child there is no background check etc). So hopefully you will be done much faster than that. the second parent can also go to Canada with the baby and at the border explain that the PR is already in process (show the proofs of paying the fee receipt etc), and most likely the baby can come in on a tourist visa (valid 6 months, and can be extended while staying in Canada). if the visa is refused, then you can apply for a temporary residence permit for the baby, based on the case of being denied etc.
You could also do the same by having both parents moving with the baby and hoping that the border officer will believe that you will apply for PR as soon as you are in Canada ( might be worth travelling with forms completed to it's more credible), and hoping that the border officer will let you in with the baby. Then once in Canada, sponsor outland as explained before.
It also depends on how long you have been out of the country - make sure that you fulfill the residency criteria for Canada, otherwise it will become more difficult.
Good luck,
Sweden
