Take them on the River bus from Parliament to Greenwich, not 'really' cheap but good hop-on hop off value: Lots to see en-route and as others have said Greenwich Maritime museum is great, www.thamesclippers.com/booking

Then after you have been to Greenwich take them through the foot tunnel to the Isle of Dogs for a giggle, walking under the Thames! And get the riverbus back from the other side of the river...cool eh...but check the pick up points! www.royalgreenwich.gov.uk/info/200259/...