Sitting here in my office in Bethesda. Several thoughts.
As noted above, Bethesda is loaded with all sorts of restaurants. I second the recommendation of Rio Grande as a kid-friendly place.
Bethesda is right on the Red Line of the Metro (there is a stop at NIH and also in Bethesda), which stops at the Zoo. It's fun to visit anytime, and they have ZooLights going on through the holiday season:
http://nationalzoo.si.edu/ActivitiesAndEvents/Celebrations/ZooLights/default.cfm
The Zoo is free during the day but as you can see on the website ZooLights has a charge.
The national Christmas tree is worthwhile. If you are driving I've found spots available a couple of blocks west of the Ellipse. There are a lot of Gov't office buildings there and, it seems to me, more available street parking there than on the east in the Federal Triangle area. If you are riding Metro I would not bother to transfer from the Red Line to the Blue Line to get to the Federal Triangle stop--just get off at Farragut North (then walk down through Lafayette Square to see the North Front of the White House, then around the White House to the Ellipse) or Metro Center.
Consider the U.S. Botanic Garden, which has a really neat holiday display with lots of model trains:
http://www.usbg.gov/upload/Holiday-08-release.pdf
I would switch at Metro Center to the Blue Line to the Capitol South station to get to the Botanic Garden, which is right at the foot of the Capitol.
We've been to lots of the drive-through and walk-through light displays around the Metro area (including at the Mormon Cathedral), but our favorite is at Brookside Gardens in Wheaton (a walk-through), which is very close to Bethesda:
http://www.mc-mncppc.org/parks/brookside/light.shtm
Over in Virginia, but very accessible by car from Bethesda, there is a great holiday singalong with the Marine Band at Wolftrap on December 7:
http://www.wolf-trap.org/Home/Find_Performances_and_Events/Performance/08Filene/1207show08.aspx