A Christmas carol concert somewhere fabulous like St Paul's Cathedral.
A panto is a must (a quirky but brilliant place to go is Wiltons Music Hall tucked away in all its glory down a side street just beyond The Tower of London. 'Mother Goose' is their panto this year.
A 'flight' on The London Eye (magical in the evening for a twinkling view) and/or trip up The Shard - often kids go free with paying adult during school holiday times.
Doing Oxford/Regent Street/Knightsbridge on a bus/on foot for Christmas lights and Christmas themed department stores' displays.
An outdoor ice-rink (Somerset House, Tower of London or the Natural History Museum ones would be my personal choices).
Trip along the Thames by boat (to the London Barrier in one direction and as far as you can go in the other direction - Richmond/Hampton Court).
Or depending how adventurous/energetic you're feeling possibly a trip to Hampton Court or even Windsor Castle
A bracing walk up on Hampstead Heath gives great views over London and some cosy pubs for a meal.
If you and your DCs are 'foodies' a trip to Borough Market is a must and is bound to be full of tempting Christmas goodies to buy and eat.
As an antidote to all things Christmas, an Indian meal (usually gets a thumbs up from teens we know). There are loads of first class Indian restaurants in London, many of them in Tooting and Southall.
And Covent Garden has really upped its game in recent years and now always looks super-festive at twilight. Full of very pukka shops and more quirky shops/restaurants as you venture along Neal Street.
And if retail therapy is a given, Selfridges, Harvey Nicks and Harrods are still 'awesome' particularly at Christmas, with a visit to their respective 'food halls' a must.
That's just for starters though...