+1 for car rental and leaving it in the car park at the center. It really is safe. We did this same trip last month and the cost of the car for the day was ~£50 plus gas +$15 to park. The car also gave us the chance for a bit of sightseeing and a stroll along the Potomac upstream. It was way more convenient than metro/taxi to IAD and the bus each way.

We left when the centre closed and still had loads of time to drop the car at Avis, shuttle bus and 3+ hours in the lounge (way too long!) - we arrived just as the place was emptying from the first flight.

Two other comments ..
The lockers there weren’t big enough for our suitcases.
If there is an IMAX film on that ends after 1700 not only is car parking “free” (you pay for the film, obviously) but this also lessens the gap to wait for the plane.

And the family loved visiting - great way to end the trip, even for non geeks.