Your babies are so cute! I can see why you might need sweaters in Canada! Though I don't knit I crochet and made Sprocket some sweaters last fall after 1 trip to the vet.
I didn't think that southern CA would be that cold, little did I know! Vets are so expensive. Anyway. I got my pattern online at
www.craftser.org go to the dog patterns, sweaters etc. You have to modify them to make them small enough but once you get the back long enough then it's only the neck piece, then the belly bands. One set of belly bands I made solid and one I used velco, they have to be easy on/off and easy wash. I used ReadHeart cheapy cause they have nice colors and easy to use. Takes about one evening to make one sweater AND if you have a model who is easy to work with (I didn't).
You can build in the D ring to the sweater or just put the harness on over. But for us, Sprocket never could quite get the hang of not peeing on the sweater, so we washed them ALOT, good thing they are cheap and easy. When you make them I hope you show us. Good luck with your models though.