Thanks again for all the helpful advice.

Following everyones suggestions I have been able to change our CLT-IAD flight to a slightly earlier one that arrives at IAD at 1131, this should now give us a little extra leeway without being rushed in order to visit the museum. Another advantage is that our new flight is on UA (rather than US) so we will have a UA-UA connection in IAD so less chance of baggage going astray.

One further question: do the UA lounges at IAD have shower facilities, or are there showers elsewhere in the airport where we could freshen up prior to our transatlantic flight?