It's important that the backpack your child carries is not only cool, says the American Academy of Pediatrics, but also ergonomically correct.
Choose a backpack with wide, padded shoulder straps and a padded back.
Pack light. Organize the backpack to use all of its compartments. Pack heavier items closest to the center of the back. The backpack should never weigh more than 10 to 20 percent of the student's body weight.
Always use both shoulder straps. Slinging a backpack over one shoulder can strain muscles. Wearing a backpack on one shoulder may also increase curvature of the spine.
Consider a rolling backpack. This type of backpack may be a good choice for students who must tote a heavy load. Remember that rolling backpacks still must be carried up stairs, and they may be difficult to roll in snow.
Here's a video about how to measure hips and torso for the right backpack size.
Here are some backpack safety tips from the American Society of Orthopaedic Surgeons
And here's a company that actually sells a toy called Pete the Posture Parrot. I bet kids just can't wait to get one! :>)