I guess it isn't a 'thing to do' as in a specific attraction, but if I were there for that time frame, I'd go on an easy walking tour. Start at Picadilly Circus, walk up Shaftesbury Ave (and depending on the age and maturity of the kids consider detouring a bit north into Soho... its a fine area but can be a bit... interesting in the evening) to Charring Cross Rd, turn south and walk down to Leicester Square, then continue down Charring Cross Rd to Trafalgar Square and from their continue down to Big Ben/Westminster. In that time you will have seen some of the quintessential sights of London, all in a walk of 1-1.5 miles.