We are spending three days in the Washington, D.C. area. I have 5 children
ranging from age 4 -18. What places are a must?

The Air & Space Museum is a must for any families.

Also the Museum of Natural History. I think they have a new dinosaur skull or something that's a major attraction right now, especially for kids. Also, try to get out to the Chesapeake Bay area for a day if you can (like Annapolis, MD). It's absolutely beautiful. Baltimore's Inner Harbor features the National Aquarium, a major attraction for kids. Have a great time!

Kelsey-
traveling with 5 kids, first stop would be your neighborhood chemist for large amounts of a sedating antihistiamine like Benadryl (for the kids) and a sedative (like Valium)for yourself.
OK, more seriously -
the age range means at least one or two of the group will likely not enjoy any given activity. Consider something like one of the hop-on, hop-off tours the first day. All will get an overview of the major monuments and sights, including Mount Vernon and Arlingtom Cemetery; you can determine how long to spend at each based on prevailing conditions (tempers, not weather!) and the ride between places builds in a brief down time for all. There are a couple vendors for this service, check out prices and routes before you buy.
If you want to visit the Capitol (a priority!) plan it for the beginning of the day before everyone gets tired, hot and fussy. The Air & Space museum has something for just about everyone; it is a very cool place, the natural history museum likewise.
Do remember to tote along some water. You might appoint one or two of the older kids to help, or even get each their own personal water bottle. Filling these at the hotel will be cheaper than buying botled water at inflated prices on the street.
If you want to visit the White House, note that you need tickets in advance, picked up at the vistiros center on E street. You can also contact your elected officials' offices for help with this.
Enjoy!!

Well, since I was a docent at Air and Space for six years, I am biased towards that museum. On Saturdays, they have 45 minute family tours. Also, on both Saturday and Sunday, they have demos geared towards kids on how things fly, paper airplanes, etc. The National Zoo is also wonderful for all ages. If you want White House tickets from your Representative or Senators, you need to request them at least 3 months in advance.

my son (12 at the time) loved the MCI Center sports museum with interactive sports games, the FBI tour, ice skating at the sculpture garden on the mall (Nov), the White House tour and all the monuments.

Take the tourmobile. For one price it stops at many of the attractions. You can get off at the stops and visit and then get on the next tram. It's a great way to see alot of DC.

I agree with Seamus, that because of the age range, not all of your kids will be thrilled with every place 100%, so here's a suggestion - maybe have each child (with the possible exception of the little one) pick one place they want to go to, say for a few hours. With careful mapping out and planning, one day can be devoted going to everyone's pick. That way, they'll all feel like they contributed to vacation.

That being said, MY suggestions would definitely be the Air and Space, American History, and Natural History museums. The older kids (and adults) would be moved by the Vietnam and Korean War memorials, as well as Arlington National Cemetary. The various Presidential monuments (Lincoln, etc.) are a must, in my opinion. The zoo would be a fun place for all the kids.
Enjoy your planning and have a great time in D.C.

On a hot summer day, everyone will love the FDR Memorial on the Tidal Basin between the Jefferson and Lincoln Memorials.

Kelsey,
And when you're done touring all of the above mentioned good stuff, the new ESPNZone in DC has a lower level full of hi-tech video games where you can turn the kids loose and mom and dad can have an adult beverage!

You get a great, free view of the whole DC area from the Old Post Office Tower.

Another place we like visiting for a few minutes that kids seem to like is the statue of Einstein located right off the Mall on Constitution and 21st.

Have fun.