Fogo de Chao - meat lover's paradise is nearby at 11th & Penn... but I don't know if there's a discount for kids, or if they'd be interested - but it is heaven for carnivores.

Butterfield 9 (14th & F) has been around for awhile and is a staple for the Washington elite, but might be a bit much for the kids as well.

I agree with the other poster - if you walk over towards MCI center (or look online), you'll find a ton of places.