I love London!!
Depending on how far you're willing to go, there are loads of museums (if they like them that is!) the Natural History museum is my favourite, hut I'm a museum geek and could spend days walking round them
Hamley's (Regent Street) is usually worth a visit, but maybe be prepared to be asked for toys. It's a cracking store and they've got demos going on on each level, some toys can be tested for yourself.
Ripleys Believe it or not is near Hamleys too, never been in it though so I can't advise.
Madam Tussaud's is pretty good. There was a 4D cinema when we went, all interactive so it made it interesting.
London Eye is good if they're ok with heights.
Westminster Abbey and Tower of London? Again museumy type things, but if they're interested could be worth it.
Depending on the days you're going, I've been told the changing of the guard is really good to watch at Buckingham. I've never caught it myself, it got cancelled once and I went on the wrong days/times about 4 times
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