I found out recently why even the youngest school children need large backpacks from a teacher friend.

1. lunch box (sandwich, fruit, maybe a small juice)
2. similar box full of carrot sticks & other yummy veggies for morning break (which was called playlunch in my day, & is now called little lunch in some/all? schools)
3. water bottle (up to 2 litres)

Poor little mites, no wonder their bags take up so much room on buses, as do the larger bags of high school students.

Years ago I saw a very small kindergarten boy being led into to a fancy school wearing a backpack meant for senior students, the bag almost covered his lower legs! The school had started enrolling juniors & soon offered a smaller bag for them.