One thing's for sure: unless they're planning on coming home on November 6, they will not be voting if they don't sign-up for an absentee ballot.
Below, I've included links to three sites with information on obtaining a ballot from Pennsylvania, New Jersey and Delaware, courtesy of the folks at longdistancevoter.org. You can copy and paste the link in an email to your son or daughter.
Each site has a link your kid can use to verify that they are, indeed, eligible to vote via mail. Then, there's the ballot application form they download, fill-out and mail back to their home-town election officials.
Once the actual ballot is mailed to them, there's the small matter of actually filling it out and returning it. But if they've gone through all the previous steps to receive the ballot, they're probably motivated enough to see this final step through.
There are plenty of deadlines, however. Each state differs slightly, but in general, you have until mid-October to apply. Ballots are due back by the Friday before the election (that's November 2 this year). Late ballots get tossed, so tell your kids not to dawdle.
And by the way, if they haven't gotten around to registering, you can also begin that process through these links, but the deadline is coming-up pretty quickly. In Pennsylvania, for example, it's October 9.