The National Building Museum offers a range of interesting objects, from drawings by famous architects to a full-scale, sustainable house built in our own galleries. Exhibits have ranged from the "art of drawing architecture" to "visions for affordable housing." Check out the museum's Building Zone, a hands-on introduction to the building arts designed especially for kids, ages two to six.
if your son enjoys legos, blocks, buildings, etc. this is perfect. Huge foyer on the main floor with space to have your own picnic. Nice restaurant inside the museum. Parking is a little easier than most museums. Current LEGO display is amazing. All confined to one room. And there is room for kids to build and display their own towers.
As far as museums go, this isn't much of one. But it's actually a fantastic play area for kids from 2-6. The carpeted atrium is one of the largest indoor spaces in DC, and has hosted inaugural parties. There are lots of building blocks and stuff for kids to stack (and knock down) at one end of the atrium. There is also a separate room called the Building Zone with more building materials, dress up costumes, make believe house, books, and more. Everything is free but there can be a line to get into the Building Zone. Parking around the museum is limited. There is a small snack shop with drinks, cookies, and fruit.