The last thing you want is to think that hauling a backpack around day-in, day-out is going to harm your child by causing pain and discomfort. The general rule is that backpacks with wider, padded shoulder straps put less burden in these areas, especially if your child is likely to be carrying heavier loads such as multiple school books.
If you’re particularly concerned about comfort, it’s also worth looking out for backpacks with additional padding in the back area which will help protect your child’s back from any discomfort caused by hard objects inside the pack.