As a photographer, I can speak to the shoulder pain that comes from putting a relatively large load (my 'work bag' is about 40lbs) for several hours - and the relief that comes from having a carrying system that puts a lot of that load on my hips (hiking pack with padded wait belt). I wonder why I've never seen book bags with weight redistribution hip belts. The few lower straps I've seen on kids' book bags were for side-to-side stabilization, not moving weight to the hips

When a backpack just isn't practical, I have found the OpTech 'SOS' (save our shoulders) straps make a heavy messenger-style bag easier to carry. The straps have just enough give that they absorb the impact of my shoulder going up and down as I walk. Worth a look if you (or your kid) has a bag with a replaceable shoulder strap.