London takes a lifetime to see, to be honest.
But do NOT waste your time on Madam Tussauds!
The obvious places are Tower of London and Westminster Abbey, I would say.
Plus your choice of museums and galleries from the National Gallery, British Museum, V&A, Science Museum, National Portrait Gallery, Tate, Natural History Museum.
Probably not worth trailing up to Camden. Try Covent Garden/Seven Dials for shops, maybe prior to a show. Avoid Oxford Street - it's awful.
But what are your/their interests? If drama, try Shakespeare's Globe. If sports, tour a football stadium, Wimbledon tennis or visit the Olympic Park. I mean, you just don't have time to do more than scratch the surface.
Plan what you do and when - the museums of South Ken are a long way from Greenwich, for instance. Book in advance, if possible, especially for popular exhibitions. Allow time to travel, and to eat, relax etc.
Then, when you get home, start planning the next trip to tick a few more things off the list.