London Eye, London Aquarium, then a boat ride/ river trip on the Thames. All within 5 mins of each other will use up one of your days. Next day Visit Buckingham palace and then get a bus down to Harrods to eat in the cafe there. After that buy a 1 day travel card and ride as many busses or tube trains for ￡12.
Tower of London, British Museum, London Eye, Buckingham Palace, Tate Museum, Globe Theatre, Victoria and Albert museum, Kensington Palace, Speaker's Corner in Hyde Park, St James Park....
You will run out of time before you run out of things to do.
That totally depends on your tastes and interests. Buy a good guidebook to London or see
www.timeout.com/london
What are the 3 siblings actually interested in?
London has everything - art, culture, museums, architecture, theatre, music, tourist attractions, parks, shops, palaces... without knowing what you might like to actually see, it's a little difficult to provide specific recommendations.
Pub crawl.
