Not Quite Dead...Edit
Robbie doesn't die in the film, but by all rights he should have. After the Ferriers narrow escape from the Hudson River, they end up in the countryside where a battle is taking place between the US Army and the invaders. Robbie tries to see the battle close-up and Ray tackles him to the ground trying to stop him. Seemingly enraptured by the carnage taking place just over the hill, Robbie keeps urging Ray to let him go, stating that he "has to see it", even though there is no rational reason for him to want to see it. Ray is forced to let Robbie go, however, when a middle-aged couple try to take away Rachel, who stands by a tree. Ray runs back to Rachel, then looks back to see a heavy explosion followed by Jeeps rolling down the hill in flames and absolutely no sign of Robbie. A tripod then appears at the top of the hill and Ray loses any hope that his son is still alive.
Despite the utter hopelessness of the situation and the odds of his survival being negligible to downright impossible, Robbie appears alive and well at his grandparents' house in Boston along with his grandparents, his mother Mary Ann and her husband Tim. No explanation is given as to how he survived, he and Ray just greet each other tearily and hug each other in front of the house.