Capitol Grounds 3. East Potomac 5. Glen Echo Park 6. Rock Creek Park 7. The Capitol grounds have a lot of nice space for kids and families to run around, play ball, have a picnic, etc.
Most areas are far from traffic and safe for energetic kids.
The view from the Capitol building balcony overlooking the National Mall is very nice and great for pictures with the family. The Capitol Building interior is open late some nights during the summer and is worth a visit. There are military band concerts on the lawn many weeknights during the summer. Don't miss a visit to the natural spring on the southeast side of the Grounds. Kids love the fountain, the flowing spring, and the cave-like structure surrounding it.