In reality, these numbers are incredibly good, so much that chasing after any improvement is bound to be extremely expensive. The money that would be spent on seatbelts or whatever could obviously be invested to greater effect in some other effort.
Given these things, I would have to disagree with you, in this case it would be worth saving the final 6 lives.
Improving general road, pedestrian, and bicycle safety would be a far better preventative measure (or even offering more/more convenient school bus service) using any money that would fund seat belts, as car accidents are the number one killer of children.
We could install those seat belts on every bus in the nation, but that would cost hundreds of millions (and the article quotes information indicating that it might actually be counterproductive anyway, but set that aside for this argument). But that money could instead be invested in finding a cure for some childhood disease, or building a poison treatment center, or better law enforcement to keep some drunks off the road, and on and on. It seems to me that any of my suggestions are likely to save more than six lives a year, so why would you want to invest that money on something that is going to do less good?