One size does not fit all. The bottom of the backpack should align with the curve of the lower back--and not more than four inches below the waistline. The shoulder-strap anchor points should also rest 1 to 2 inches below the top of the shoulders.
Inspect before you invest. Look for sloppy stitching or loose threads (which can indicate poor manufacturing); fabric edges that can fray and weaken the fabric, and possibly get stuck in the zipper; and zippers without fabric flaps covering them (which can allow water to seep in).
Comfort, convenience, and safety are important. Features to look for: wide, padded shoulder straps to distribute the weight over a large area of the shoulder; an abdominal strap to distribute the pack's weight evenly on the back, waist, and hips; and reflectors or reflective fabrics to add visibility at dusk or dawn.
Prevent injury with a lighter load. Load and wear it correctly--the American Occupational Therapy Association recommends carrying no more than 15 percent of your body weight in a backpack. The less your child carries, the better.