Good spinal health starts at birth. Here are a few observations you could try to make regarding your children's posture and gait:
do their feet (one or both) turn in when they walk?
do they have a head tilt?
if yours is still a baby, are they crawling properly or do they scoot along on their bottom?
is theirs a commando crawl and not hands and knees?
These are all signs your baby/child may have misalignments in their spine which will only worsen with time. The body compensates for these tilts and turns by creating new tilts and turns. They do not resolve on their own.
A tip on Backpack use: Please make sure your child's backpack is snug to their back and the top of the pack is at the top of their shoulders.