Slate has an article up with ratings on kids backpacks. The main angle here is for school, but day packs were originally intended and as far as I am concerned, still have the primary use of supporting day hikes.
Be assured there are many more packs out there than described in the article. All of the big outdoor companies such as The North Face and Kelty are selling them too.
It used to be that "offshore" packs were made with inferior materials, but in this "globalized" world, pretty much every pack is made offshore , and the materials that used to be exclusive to the US are now used wherever the packs are made. This means that most of these products nowadays are of decent quality, regardless of where they are made. My experience tells me that these packs are also made stronger because of the significant loads of books that kids carry. That is a whole different topic.
My favorite day pack until it literally fell apart last year was a circa 1986 Chouinard Equipment daypack. Lightweight, no frills, strong, doubled as a sleeping bag stuff sack. No pictures left of that one.
For those looking for deals, I always check this site.
