Map of the National Mall

Most major attractions in Washington, DC are in and around "The National Mall", which is the area in Green on the map below.  The Lincoln Memorial is on the West side of The Mall, and the United States Capitol building is on the East side.  Those two structures are separated by over 2 miles, and it takes over 45 minutes to walk the length of The Mall, so plan accordingly; especially if you're touring with children.

An excellent touring plan would be to visit attractions at one end of the The Mall on Day 1, and the other end of The Mall on Day 2.  For example, visit the Lincoln, Jefferson, Vietnam and WWII Memorials (West side) on day 1.  Then visit the Smithsonian Museums such as Museum of Natural History, Air and Space Museum, plus the Capitol Building (East side) on day 2.