Also - I don't know if you want to infuse your girls with sugar.... if you do, they might enjoy a stop at Georgetown Cupcake and/or Cakelove - both have gotten a lot of media coverage, and TV shows dedicated to them. Be aware that there is often a long line for Georgetown Cupcake. Neither really has a place to sit - grab and go.
http://georgetowncupcake.com/ http://cakelove.com/ There's both a running store (Georgetown Running) and a cycling store a few blocks down from Georgetown cupcake. You could always eat at the Tombs, check out the Exorcist stairs, and then indulge in the running and cycling stores before hitting Georgetown Cupcake.
You can hit U Street for both Ben's Chili Bowl and Cakelove - your arteries will not thank you after that meal.
Also (can't believe I forgot this before), check out Busboys and Poets for one of your meals - it's a modern DC institution, and also should be budget friendly.
http://www.busboysandpoets.com/