Uber would be a good choice for Mt Vernon to go earlier. It's super easy using app. It's possible to navigate everything w/ transit & Metro is great.
Monuments are a must. Couple non-mall museums-- the Natl. Geographic one is pretty cool and not far from embassy row which might be interesting. At that age, the most popular will probably be Air & Space, Natural History, & American History. Holocaust is moving and pretty emotionally draining. I'd do it on a day with some low key stuff afterwards. I'd take them to Ford's Theater too w/ Lincoln museum in basement. You can just look in rotunda of Lib. of Congress & it's worth it.
There are a lot of outlines for sites to hit w/ a family. Love this city.