Two StrapsMake sure the bag has two straps. Single strapped bags, like satchels and duffel bags, should be avoided. A single strap places the entire load on one side of the body. Two straps balance the load on both shoulders.
A single strap bag is sufficient for carrying light loads, like a change of clothes, but should not be used to carry any real weight.
SizeThe second most important thing is to have the right size bag. The bag should be no larger than the child's back. It should rest 1-2 inches below the shoulders and no more than 4 inches below the waistline (note: the waistline is level with the bellybutton).
Wide, Padded StrapsThe bag should have wide shoulder straps. Wide straps distribute the load over more area of the shoulder. The wider the better, with a minimum of two inches.
The straps should also be padded. Padding spreads the load as well as alleviates any pressure points.
Padded BackThe back should be padded as well. It will alleviate any pressure points. And it will protect the back from being pocked and prodded by what is loaded in the bag.
LightweightThe bag should be light. The lighter the better. The stress on the back is caused by the weight of the bag. Anything you can do to reduce that weight will reduce the stress.
Waist StrapA waist strap is a wonderful thing. It dramatically helps direct the load away from the shoulders and onto the much stronger waist and hip muscle groups.
By lowering part of the load to this point you also remove it from the spine. Less stress on the back means less chance of back pain.
CompartmentsHaving a bag with several compartments helps in two ways.
First, it helps keep the load where you put it. A properly loaded backpack can go a long way to reducing the stress. Furthermore, a shifting load mean a shifting (dynamic) stress. Dynamic stresses are bad.
Second, it lets you put flat things next to the back and pointy things away from it.