The reason the bus driver has a seat belt and the kids don't is because the kids have a padded seat back in front of them to stop them from launching forward in a crash. The bus driver has nothing but glass and the open road in front of them to stop them from launching forward in a crash. And seat belts help protect the bus driver from the airbags as they deploy from the steering wheel which have been known to deploy so forcefully that if you're not wearing a seat belt they can kill you, and even in some extreme circumstances completely decapitate you.
Also as someone else pointed out the kids could get trapped in their seats in the event of a fire. The bus driver has a little seat belt cutting tool available to them, but in a fire they might not have time to cut 72 seat belts to free all of the kids on a big bus.
You might ask, well what if the bus rolls? It's pretty unlikely that the bus would roll because bus drivers are trained pretty extensively and have to go through periodic medical exams and driving exams to make sure they're capable of doing the job safely. Even if the bus were in a situation where it might roll, it's very bottom heavy so it would take quite a lot to get it to tip over.