Virginia Beach is way too far for a 2-day road trip from Philly. If you want beach, the southern NJ or Delaware beaches are the place to go--the weather should still be pretty good the week after Labor Day, and prices will be down (although if you want to stay overnight I would advise you to book lodging immediately).
Lancaster County and/or Gettysburg would be great places to hit, particularly with a child, if you have a Philly home base. You can do Lancaster on Saturday (most things are closed there on Sunday) and continue on to do Gettysburg the next day, if you want. It's about a 2.5-hour drive from Gettysburg back to Philly.