forget usa or canada.
you are chinese citizen, your spouse is too (both holding valid china passport), you 2 just have immigrant status in usa/canada, now you 2 want to let the kid (born in ca or usa) go back to china and stay with grandies for a period of time.
my humble undeerstanding is: the kid is a Chinese citizen naturally, but the embassy or consulate policy does not tell the same story. it will be hard for the kid to obtain chinese passport, this has nothing to do with 寄养, i just talk the travel. :-(