Implying that Mario did the Koopa kids in for good is too violent for a Mario game? Landing on living creature's heads and crushing them flat isn't basically killing them in a borderline slasher flick kind of way? Or smashing in a being's skeleton with a jumbo sizeed hammer isn't violent? Its all the same to me, the only difference is that the Koopa kids have identities, rather than being random Goomba's that no one gives a rip about. Besides which, they're really only called "The Koopa Kids" because they're Bowser's offsrping. We don't actually know that they're really kids, persay. They could just be young adults. Its only in the SMW TV series that they're called kids, but I'd hardly call that canonical (just look at their inpertretation of Link from "The Legend of Zelda").
that being said, the good ol' Koopa kids have WAY more personality than Bowser jr. (shudders).