My beef with commercial kids' backpacks is that those packs weigh 2 1/2 to 3 lbs, which is close to half the load that the average 6-year-old should carry.
On the other hand, I've found (through experience with my grandkids) that kids do need a supportive pack frame. The #1 complaint, at least with the three grandkids, is the pack's putting too much pressure on the shoulders. IMHO, a good frame and load lifters are pretty essential. I finally got my oldest grandson a Deuter Fox and he (at age 9) could carry 12 lbs. with no discomfort. Still, he could have shouldered a bigger share of the load if the pack body had been a pound lighter.
On the other hand, a commercial pack (like the Deuter Fox or the REI Comet) with a good supportive frame has over half its weight in bomber-proof nylon, a zillion pockets, heavy zippers, daisy chains and many other gewgaws. What's wrong with just a simple Dyneema pack body with lightweight compression straps and a couple of mesh outside pockets? I wouldn't want to use silnylon or cuben with a child; on the other hand I'm a strong believer in teaching children to care for their belongings. They should be supervised enough that they don't drag the pack over rocks or throw it off a cliff!
I'd be wary of making a pack with insufficient support. What I'd do, if I could sew, would be to find a good supportive child's pack on sale, completely remove the pack bag and incorporate the frame into a simple pack made of Dyneema. That would give you the best of all possible worlds--a highly supportive frame and a lightweight pack!