Drop your bags at the Marriott Washington Metro Center, which is centrally located and offers in-house babysitting services, complimentary cribs, and a children's menu.
Take the gang straight down to the Air and Space Museum; you'll want to arrive early to avoid lines, as this is a very popular attraction. Don't miss the How Things Fly exhibit, a hands-on educational display with a fighter plane flight simulator, and catch a show in the IMAX theatre; it's a great way to introduce the science of space travel to young minds.
Next walk just down the block to the Museum of Natural History. The Butterfly Pavilion is a fantastic spot for group pictures, and the dinosaur skeletons will shock you with their size. The museum has two cafes serving fresh, healthy food, so grab lunch here instead of the Air and Space Museum, who only offer fast food.
In the afternoon get everyone out into the fresh air with a trip to the National Zoo to say hello to the pandas. Take the red line on the Metro to get there, exit at Woodley Park station. The zoo is large and hilly, so be sure everyone is wearing comfortable shoes, but they rent strollers if anyone gets cranky.
For a dinner closer to the hotel that will please the whole family, head to Casa Luca in Mount Vernon Square. It's a welcoming, relaxed spot where the grown-ups can sip wine while the kids draw with crayons. The food is simple family-style Italian, but still sophisticated enough for an adult palate.
Arrange a sitter at the hotel, then walk a few blocks north for after-dinner drinks at Jaleo. The lively atmosphere and fun cocktails will energize you for the next day's adventures.