The Bottom Line: The weight of the backpack should not exceed 15% of the weight of the kid.
That’s the guideline. But the issue raises the issues of posture and strength in kids. Both should be addressed by parents and by their chiropractors.
We weigh the bags (some are ridiculously heavy). Often in doing so we find that the backpacks are not well built, or not adjusted properly, or simply don’t or can’t be fitted to the child well.
It’s something that is easy to fix and is sometimes a big deal. No need to let the kid suffer because of something so easy to take care of.