Not really sure if this is too babyish, but how about this:
http://cbhstudio.com/jojo-dog-backpack-p13.html
Man, I looked everywhere I could think/searched all sorts of terms. Its amazing how little there is for dog backpacks.
Well, if your daughter decides she wants a standard backpack, lol, I found this
20% off backpacks at The Childrens Place coupon.
Here's a link to their backpacks (you have to scroll all the way down).
Best of luck!