Well Penny isn't a child of the island like Alex. It was Alex's destiny to die, which was only delayed.
Well, obviously Alex (born ON the island) is not a "child of the island" like Locke (NOT born on the island) is.
Locke was resurrected, Alex wasn't.
As for Penny, Ben's not killing her shows a continuity in his character--having lost his mother, he does not want to inflict that pain on another child.
I like Penny and Des. If Des is special to the island (it's laws don't apply to him), then maybe his kid is also special to the island?