A good spot to let the kids get their wiggles out. 

Although it's not an obvious fit, the Hayes Senior Wellness Center, located between the NoMa and H Street Corridor neighborhoods, has a small playground on its grounds.  At the corner of 5th Street and K St NE, this playground is small with a fairly standard play structure best suited for younger kids.  It has ladders, slides, and a bridge connecting to the two sections.  The playground is enclosed.  While it's not terribly exciting, if you're in the neighborhood and need to let the little ones burn off some energy, this playground will do the trick.  If you want a bite to eat or a coffee, head to the nearby La Colombe coffeehouse or Union Market.

The closest metro is Union Station.