Hopefully you're aware there is a residency requirement in order to maintain your permanent residence status: In order for your permanent status to remain valid, you must live in Canada for at least 2 years of every 5 year period after your landing. Apparently you landed sometime around Feb 2007, but you don't say how long you lived in Canada before you left. So it's difficult to know whether your PR status is still valid. Simply coming back before it expires is no guarantee it's still valid - if you have been gone in excess of 3 years, it won't be.
Assuming you lived in Canada for almost 2 years after your landing, you should be okay - and, in that case, sponsoring your child is what you'll need to do. Unfortunately, CIC is no longer posting dependent child sponsorship timelines separately from regular family class sponsorships - so it's not really possible to give you a specific timeframe. I would guess-timate 6-9 months, though, considering that the dependent child processing used to take less time than spousal processing, and that's now at 11 months through Buffalo. There is no inland processing for a dependent child - so the case would be processed through Buffalo, even if the child is in Canada with you during processing. However, one thing, don't assume that just because s/he is a US citizen (and, therefore, visa-exempt) s/he will be allowed to enter Canada with you - especially when the intention is clearly to be here in order to apply for the child's permanent status, when you haven't been here up until now. And, depending on your personal timeline - how long you've been away from Canada during the five year period after your landing - it could end up being a huge complication if one of you has to stay back and wait for the child's PR to be finalized - because, in doing so, they could risk losing their own PR status because of not being able to return to Canada in time to fulfill the residency requirement. We'd be able to tell for sure if we knew more specifics - but you can probably figure it out for yourselves, too.
