Great place for kids
This hotel is very nice. The rooms are large and clean. There is a fold out couch in a separate room. There is a small refrigerator in the rooms. Breakfast is perfect for the kids - a buffet will all the usual stuff - waffles, pancakes, eggs, bacon, cereal, etc. At 9 and 6 they give out fish food for the kids to feed the many koi. 
DC Trolley picks up right across the street at the Marriott. 
One downside is that this hotel is about 4-5 blocks from the nearest metro stop (dupont circle). It is about a $10-15 cab ride to get to the monument/museum areas. Another issue is that the elevators can get pretty crowded and you might wait a while for one during busy times. 
Good kid restaurants in the area - Firefly, Grillfish, etc.