Are we all talking about the same thing, here?
The original post said the kid was in a box, in the back of the truck.
Many of the responses are about how "I rode in the box all the time", and y'all are talking about the bed.
As I read that original post, the kid was in a box, that was in the bed.
I rode in the bed of a pickup, in high school. Didn't hurt me.
I carried my kids in the bed of my pickup for years.
I see nothing wrong with riding in the bed - sitting on the deck, or on the wheel-well.
But, several years back there was a local story. Little kid. She was in a plastic wading pool, in the bed of the pickup. And somehow that plastic wading pool took flight. Pool, and little girl, landed in the highway, where she was killed by another car.
I don't think there would have been a problem, if she had just been "in the bed". But being in that pool, with all that solid surface area to catch wind. Hell, maybe his tailgate was down (story did not say whether it was up or down) and the pool bounced out. Would not have happened if it was just the kid on the bed floor.
So - if it's some little kid, in a BIG box (like, maybe, an appliance box - washer, refrigerator) - that BIG box could leave the truck bed much easier than just the kid would.