Budget Travel has recently released a list of their top 15 places children should see before they reach the age of 15. This is an updated list from their previous one released back in 2009. Many of these sites have historical significance, like Ellis Island and Monticello, but not all of them.
Some of the places listed are far more fun than educational, like Walt Disney World and Fenway Park. While any of them would make wonderful family vacations, only a few of them are appropriate for younger kids, like the San Diego Zoo. While they could still be dragged through the Redwood National Park, they may not appreciate as much when they are younger.
One site that should definitely wait until the kids are older is Alcatraz Island, especially if you choose the night time tour. Some kids may find the fact that they are on an island that is rumored to be haunted a little too frightening; stick to Disney instead.