I don't think the National Zoo is on the trolley route. I have lived in the DC area almost my whole life, so we drive everywhere and I have never taken a tour bus or trolley. The Air and Space museum, Natural History museum, Washington Monument, US Capitol, Jefferson and Lincoln Memorials would be my picks in terms of location to each other and to keep the interest of 8 year olds. Also, if you're looking to eat somewhere besides the museums, there are places a couple of blocks off the Mall area - near Spy Museum (McCormick and Schmick's among others). You can't see it all in a day, but you can see a lot if you don't try to read everything. Also, if you had more time you could take the bus out to the Udvar-Hazy Air and Space museum near Dulles airport - I pass it everyday on my way to work.