A well designed kid's pack won't be just a smaller version of an adult's pack, it will be shaped to fit a kid's narrower shoulders and hips. Look for packs that have padded backs, adjustable straps for the shoulders and hips, and webbing that's narrow enough that it won't dig in or chafe against their skin.
For some older children, a small adult's pack might be the right size if the torso length is correct. To determine your child's torso length, measure in a straight line from the most prominent vertebrae at the back of the neck (about the same level as the top of the shoulders) to a point on the spine that's level with the hipbone. Compare this measurement with the manufacturer's given back length (top to bottom of the back panel).
Encourage your child to carry their pack using both straps. When correctly adjusted, the bottom of the pack should rest near the hipbone or top of the pelvis. (Many children tend to carry their packs too low). The shoulder straps should not pinch or squeeze, and they should keep the load close to the child's body. The hipbelt, if there is one, should rest directly above their hipbones.
Most experts recommend that kids don't carry more than 10 to 12% of their body weight. So if your child weighs 40kg, they shouldn't carry more than 4 to 5kg in their backpacks. Lots of pockets and compartments will help distribute the load, but the heaviest, bulkiest items should rest close to the spine.
Kids' gear should be designed to take some abuse. Look for these features that indicate a well-constructed pack: