The Smithsonian is a must. The Air & Space Museum is a must. The Jefferson Memorial is a must- a good place to take your lunch and catch your breath and look back at it all. You have to walk the Mall. The Capitol is cool but if the kids are so young it will probably be enough just to take them up the steps- same with the White House- a walk-by may satisfy them. The Washington Monument is a favorite- it involves an elevator ride to top.