3 Nights in D.C.
Here are a few places I think you'll enjoy with the kids:
Chili's, TGI Fridays, and McDonald's.
Ha. Just kidding.
I think Oyamel would be pretty cool. The food is fantastic, and they have folks on staff hand making tortillas and such. Penn Quarter, which is a fun location. Not can't miss per se, but...
Hank's Oyster Bar in Dupont would be good too - great food, nice ambiance. But we are just past the season for oyster, aren't we?
Central would be great with kids. It's can't miss, and relaxed with a approachable menu. Actually, this is probably your best bet.
You could also try Matchbox. Not quite can't miss, but a solid, solid place. Also in the Penn Quarter.
I mean, the can't miss places aren't quite kid appropriate: CityZen, Citronelle, Eve in Alexandria, and The Source.
Don't go to Hook. Do some research off this board to understand why (try Washington Post chats by food critic Sietsema).