If we're so all-fired concerned with the calculus of saving children's lives, we shouldn't be banning "assault rifles", we should be banning residential swimming pools and hot tubs. They kill more children 15 years and younger each year than rifles of *any* kind. Plus, it would be much easier:
1) No one "needs" a swimming pool or hot tub - they're of very limited usefulness other than a luxury/recreational item,
2) There are no pesky Constitutional protections for owning a swimming pool, and
3) It would be a heck of a lot harder to hide an illegal pool or hot tub than it will be to hide an illegal rifle.