Gonna need you to flesh this one out for me. Why would a heavy backpack be healthier? They tend to promote bad posture and back/neck/shoulder problems. The extra weight resistance during walking probably isn't going to outweigh the potential negatives.

https://www.nbcnews.com/better/health/why-bags-are-wrecking-...

I cannot vouch for the accuracy, but I've heard it speculated that heavy backpacks are bad for kids backs.

If you think kids need more exercise, increase the time spent in gym class/phys-ed.