There's not much in Georgetown that would interest a 3 and 7 year old, other than perhaps walking along the riverfront? However, there are other, better things that will be accessible to you if you have a car - like Theodore Roosevelt Island. If the kids like nature, pack a picnic and head over there (you have to drive into VA to get there unless you go by boat, but the island is technically in DC). There are walking trails, marshland, and good views across the river from the ends of the island.

Along those same lines, the National Arboretum in Northeast DC is very pretty and also has lots of open space if the kids need to let off some steam. Keep in mind that the location is somewhat off the beaten path and can be difficult to find, so this probably isn't worth the haul unless you have a lot of time to kill...there's nothing else even remotely interesting to a tourist near it.