LaraK has good suggestions. Basically, start at one end of the Mall and walk down to the other end, stopping at those places you want to hit. You can spend as much or as little time as you want in each place, depending on your family's interests and tolerance for walking.
Couple other places you might want to stop briefly on the Mall, depending your your family's interests
* Botanic Garden is right next to the Capitol, neat building and plants, lots of good places to sit and rest/look.
* Museum of the American Indian - between Botanic Garden and Air & Space. Fantastic display of arrowheads and hand made beaded items. Cafe is one of the better places to eat on the Mall.
* outdoor sculpture garden of the National Gallery of Art. This is a great place for kids, some of the sculptures will definitely appeal to them. Large fountain in the middle with lots of nice seating, another place to rest or eat those hot dogs LaraK mentioned. Also has a decent bistro, albeit kinda pricey. Right in front of the Archives...
* Archives IF the line to see the Bill of Rights, Declaration of Independence is short. Wouldn't wait too long for this given the ages of your kids.
Definitely hit the monuments. Good suggestion to see them at night. Other than a bus tour that drops you off relatively close to them, there's no good way to see them other than walking.
Wouldn't use your limited time to see either the Zoo or Holocaust Museum, they just take too long to see or travel to.
Walking is the best way to get around the area, and you'll probably walk as much or more than a day at WDW. Strollers are not nearly as common as at WDW, they really slow you down going into buildings and using Metro.
HTH and you have an enjoyable but exhausting day in DC.