In "The End", there are some theories mentioned about what happened to the children when they left the island. One suggests that they went down with the ship as it sank. Another suggests that, despite the ship sinking, they made it to the shore, eventually changed their names, and possibly joined V.F.D.
In "The Beatrice Letters", it's hinted that the children are alive, although their exact whereabouts are unknown, hinting that the latter theory is the more canonical possibility.
With that said, what do you think their aliases could be?