It didn’t dawn on me until Patrick’s comment on Greg’s last Lost post, but I’ll bet that little Charlie, Penny and Desmond’s child, is in fact Charles Widmore. Little Charlie will be whisked to the past on the Island where he can then grow up to be the Charles Widmore we know. He is, as the song says, his own grandpa.
The reason why Ellie and Charles are so determined not to change the course of events is not that it is impossible, but because it allows Charles to exist. So Ellie goes around convincing people, including Desmond and Daniel, that time cannot be changed because she doesn’t want time changed.
I’d further guess that the coming war will be between those who are trying to change the course of time and those who believe that it can’t or shouldn’t be changed. Jack will lead the change faction and John will lead the destiny faction. That’s why Charles Widmore needed back on the island to ensure that the right side is going to win.
Update — And I’ll bet that Charlie has already been kidnapped by Charles and Ellie. The nurse oddly told Penny to leave her child while visiting Desmond. You wouldn’t bother having that bit of dialogue unless it mattered.
(edited to add link, correct typo, and add update)